T  N 

1G7 


IRLF 


32    3b3 


BULLETIN 

OF    THE 

DEPARTMENT  OF  MINING  AND  METALLURGY, 

UNIVERSITY  OF  CALIFORNIA. 

THE  ELECTROMOTIVE  FORCE 
OF  METALS 

IN  CYANIDE  SOlllTIONS. 


BY 


S.  B.  CHRISTY, 

i 

PBOFKSSOR  OF  MINING  AND  METALLURGY,  UNIVERSITY  OF  CALIFORNIA,  BERKELEY,  CAL. 


A  PAPER  READ  BEFORE  THE  AMERICAN  INSTITUTE  OF  MINING 

ENGINEERS,  AT  THE  CALIFORNIA  MEETING, 

SEPTEMBER,  1899. 


REPRINTED  FROM  VOL.  XXX.  OF  THE  TRANSACTIONS, 
BY  PERMISSION  OF  THE  COUNCIL. 

1900. 


GIFT  OF 


[TRANSACTIONS  OF  THE  AMERICAN  INSTlfUT fiV^  MINING   ENGINEERS.] 


The  Electromotive  Force  of  Metals  in   Cyanide  Solutions. 

BY  S.  B.  CHRISTY,  PROFESSOR  OF  MINING  AND   METALlURGY,  UNIVERSITY  OF 

CALIFORNIA. 

(California  Meeting,  September,  1899.) 

THE  practice  of  the  cyanide-process  of  gold-extraction  has 
brought  to  light  many  important  contradictions  of  familiar 
chemical  analogies,  which  etill  obscure  both  the  theory  and  the 
practice  of  the  art  with  distinctions  subtler  and  more  difficult  to 
make  or  follow  than  those  which  delighted  the  heart  of  the 
old-time  metaphysician.  Yet  Nature  herself  has  drawn  these 
distinctions ;  and  if  we  hope  to  succeed  in  this  modern  search 
for  the  Golden  Fleece,  we  must  be  able  to  follow  her  through 
the  winding  labyrinth. 

There  are  so  many  phases  of  this  question  that  I  shall  attempt 
to  touch  on  only  one  of  them  at  this  time,  but  it  is  one  that  lies 
at  the  root  of  many  others. 

In  reviewing  my  paper  "  On  the  Solution  and  Precipitation 
of  Cyanide  of  Gold,"*  Mr.  E.  B.  Wilson  contendsf  that "  in  the 
solution  of  gold  by  the  means  of  alkaline  cyanides  the  various 
reactions  are  determined  as  to  their  order  and  intensity  by  the 
relative  positions  of  the  elements  concerned  in  the  electro-chem- 
ical series,  or  series  of  voltaic  tension." 

In  a  modified  form  this  statement  is  probably  true.  That  is 
to  say,  the  difference  of  electrical  potential  in  any  closed  elec- 
trical circuit  determines  the  nature  of  the  reactions  which  en- 
sue. But  the  matter  is  not  so  simple  as  Mr.  Wilson  assumes.  Re- 
cent investigations  show  that  the  order  of  the  metals  in  the 
electro-chemical  series  depends  not  only  on  the  nature  of  the  ele- 
ments themselves,  but  also  on  the  chemical  composition  of  the 
solution  in  which  they  are  placed ;  its  degree  of  concentration ; 
its  temperature ;  and  in  the  case  of  gases,  on  the  pressure. 

Unless  all  these  conditions  are  taken  into  account,  inferences 
drawn  from  the  electro-chemical  series  are  likely  to  prove  more 

*  Trans.,  xxvi.,  735.  f  Trans.,  xxvii.,  821. 

1 

321305 


2  \  THE  *  ELECt RbMOTIVE    FORCE    OF    METALS. 

misleading  than  useful!"  *  The* series,  as  quoted  by  Mr.  Wilson 
from  Gore,  is  as  follows : 

3.  Potassium.  28.  Antimony. 

4.  Sodium.  i9.   Tellurium. 

8.  Calcium.  31.  Gold. 

9.  Magnesium.  37.  Carbon. 

12.  Manganese.  39.  Nitrogen. 

13.  Zinc.  40.   Arsenic. 
15.  Iron.  43.  Sulphur. 
20.  Lead.  45.  Bromine. 

24.  Copper.  46.  Chlorine. 

25.  Hydrogen.  47.  Oxygen. 
27.  Silver. 

This  series  correctly  shows  the  difference  of  potential  in  many 
solvents,  especially  in  acid  solutions,  but  the  use  of  it  for  pre- 
dicting the  action  of  cyanide  solutions  involves  several  grave 
errors,  one  of  which  is  the  assumption  that  the  nature  of  the 
solution  in  which  substances  are  placed  is  without  effect  on  the 
order  of  the  series. 

The  remarkable  effect  of  solutions  of  cyanide  of  potassium 
upon  the  relative  positions  of  substances  in  the  electro-chemical 
series  was  first  shown  by  Prof.  Jacoby,  who,  on  August  21, 
1844,  called  the  attention  of  the  St.  Petersburg  Academy  of 
Sciences  to  the  fact  that  when  silver  and  cyanide  of  potassium 
solution  replace  the  zinc  and  sulphuric  acid  in  the  Daniell  cell, 
a  strong  current  ensues  and  copper  is  precipitated.  Ordinarily, 
and  according  to  the  usual  inference  from  the  above  series,  cop- 
per precipitates  silver  from  its  solutions ;  but  here  was  a  com- 
bination in  which  silver  precipitated  copper. 

In  the  following  year,  Poggendorff  announced*  that  by  his 
(now  well-known)  "  compensation-method,"  he  had  found  the 
electro-chemical  series  in  1  part  of  KCy  to  8  parts  water — i.e., 
in  a  12.5  per  cent,  solution  of  KCy,  to  be : 

1.  Zinc  amalgamated.  9.   Lead. 

2.  Zinc.  10.  Quicksilver. 

3.  Copper.  11.  Palladium. 

4.  Cadmium.  12.  Bismuth. 

5.  Tin.  13.   Iron. 

6.  Silver.  14.  Platinum. 

7.  Nickel.  15.  Cast  Iron. 

8.  Antimony.  16.  Carbon  (Kohle). 

*  Annalen,  Bd.  66,  s.  597,  1845. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  6 

Gore  also*  investigated  this  subject  with  similar  results;  only 
he  showed  that  the  order  depends  on  concentration  and  temper- 
ature; and  that,  according  to  these  conditions,  gold  may  be 
more  or  less  electro-positive  than  silver. 

The  variations  thus  discovered  in  the  relative  position  of  the 
metals  in  the  electro-chemical  series  at  once  cast  a  cloud  on  its 
usefulness  for  predicting  chemical  reactions ;  and  though  a  great 
mass  of  experimental  data  was  accumulated,  and  the  most  acute 
minds  of  the  century  were  brought  to  bear  on  the  problem,  no 
explanation  of  these  anomalies  was  found  for  many  years. 

It  is  only  within  the  last  decade  that  anything  like  a  clue  to 
the  mystery  has  been  detected ;  and  this  result  has  been  made 
possible  only  through  the  combined  efforts  of  a  number  of  men 
who  approached  the  subject  from  what  may  be  almost  termed 
its  purely  speculative  side,  without  any  thought  of  practical  ap- 
plications. 

Xow  that  something  tangible  s-eems  to  be  resulting  from 
these  efforts,  I  have  thought  that  a  brief  outline  of  the  rapid 
progress  made  in  the  electro-chemical  theory  during  the  last 
decade  might  be  of  service  to  those  who  have  been  too  much 
occupied  with  practical  details  to  follow  theoretical  investiga- 
tions for  themselves.  Such  an  outline  will  also  render  more 
clear  the  bearing  of  the  experimental  work  which  follows. 

I. — OUTLINE  OF  THE  DEVELOPMENT  OF  THE  MODERN  ELECTRO- 
CHEMICAL THEORY. 

Analytical  Research. — In  the  development  of  this  subject,  the 
efforts  of  investigation  in  the  line  of  pure  mathematics  have 
been  combined  with  the  experimental  methods  of  the  chemist 
and  the  physicist  with  the  happiest  results. 

Chief  among  the  mathematicians  in  this  particular  field  is 
Prof.  J.  Willard  Gibbs,  of  Yale  University,  whose  work  is  too 
little  known  and  appreciated  by  his  countrymen,  or  even  by 
his  own  colleagues.  His  essays,  published  in  the  Transactions 
of  the  Connecticut  Academy  of  Science,  being  purely  mathe- 
matical, attracted  but  little  attention  in  this  country,  but,  being 
translated  into  German  by  Prof.  Ostwald,  were  introduced  to  a 
public  capable  of  appreciating  them.  He  is  now  recognized  in 

*  Proc.  Royal  Soc.,  Lond.,  vol.  xxx.,  p.  45,  1879. 


4  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

Europe  as  having  anticipated  many  important  discoveries  with 
which  Helmholtz  had  been  credited.  His  predictions  concern- 
ing the  laws  governing  matter  and  energy  have  been  verified 
as  far  as  they  have  been  tested,  and  are  even  yet  far  ahead  of 
experimental  verification.  It  is  impossible  to  give  here  an 
adequate  account  of  the  work  of  this  great  man,  but  he  will 
always  be  .recognized  as  a  leader  in  the  application  of  mathe- 
matical analysis  to  the  most  profound  physical  and  chemical 
problems. 

The  "  Ions." — While  Gibbs  and  Helmholtz  were  busy  with 
the  mathematical  side  of  the  question,  Daniell,  Kohlrausch, 
Hittorf  and  many  others  were  busy  in  following  the  experimental 
lines  opened  up  by  Faraday.  Faraday  had  always  assumed 
that  the  electric  current  was  transported  through  a  solution  by 
discrete  particles  of  matter,  each  bearing  its  own  electric 
charge.  To  these  moving  particles  of  matter  he  gave  the  name 
of  "  ions."  Those  which  move  in  the  solution  in  the  same  di- 
rection as  the  positive  electricity  he  called  "  cathions,"  and 
those  which  move  in  the  opposite  direction,  "  anions."  The 
electrodes  he  distinguished  as  the  "  cathode,"  to  which  the 
cathions  move,  and  the  "  anode,"  to  which  the  anions  move. 
These  distinctions  have  proved  of  the  greatest  service;  their 
value  and  meaning  have  been  made  yet  more  clear  by  the  work 
of  Daniell,  and  most  of  all  by  the  classic  experimental  researches 
of  Hittorf.  The  latter  showed  beyond  question  that  the  pas- 
sage of  the  current  was  accompanied  by  an  actual  transfer  of 
the  cathions  and  anions  through  the  solution  in  opposite  direc- 
tions. He  and  those  who  followed  him  were  able  to  determine 
that  these  ions  were  sometimes  simple  elements,  like  sodium, 
potassium,  silver,  copper,  etc.,  and  at  other  times  compound 
molecules  like  SQ*,  NH4,  N03,  etc.  Thus,  while  common  salt 
would  have  for  its  cathion  sodium,  and  for  its  anion  chlorine, 
sodium  nitrate  would  have  for  its  cathion  sodium,  and  for  its 
anion  N"03.  He  proved  these  propositions  by  ingenious  exper- 
imental methods  which  are  still  admired  for  their  simplicity  and 
certainty. 

Hittorf  showed  also  that,  in  the  case  of  potassium  argento- 
cyanide,  the  principal  cathion  was  not  silver,  but  potassium, 
which  alone  traveled  in  the  direction  of  the  positive  current. 
The  silver  traveled  in  the  opposite  direction,  with  the  cyanogen 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  5 

and  the  negative  electricity,  and  formed  the  anion  (AgCy2).  He 
explained  the  reduction  of  the  silver  which  takes  place  at  the 
cathode  by  proving  that  all  (but  a  trace)  is  due  to  the  second- 
ary reaction  which  ensues  when  the  potassium  ion  is  deposited 
at  the  cathode  and  displaces  an  equivalent  of  silver  from  the 
silver  cyanide  there  adjacent. 

This  experiment  is  so  important  that  it  will  be  considered 
more  extensively  below.  It  is  sufficient  in  this  place  to  say  that 
he  proved  that,  while  an  equivalent  of  silver  was  deposited  at 
the  cathode,  the  adjacent  solution  was  robbed  of  that  equiva- 
lent, and  at  the  same  time  there  was  found  an  extra  equivalent 
of  potassium  in  the  form  of  caustic  potash,  while  about  the 
anode  there  was  an  increase  of  one  equivalent  of  silver  and  two 
equivalents  of  cyanogen.  The  conclusion  is  irresistible  that  the 
principle  ions  are,  cathion  (K),  anion  (AgCy2).  He  proved  also 
that  the  ions  migrate  with  different,  moderate  and  easily  meas- 
ured velocities. 

Molecular  Conductivity. — Meanwhile  Kohlrausch,  Ostwald  and 
others  were  making  a  tedious  and  apparently  useless  investiga- 
tion on  the  electrical  conductivity  of  solutions  of  increasing  di- 
lution. The  specific  conductivity  of  dilute  solutions  is  usually 
much  smaller  than  that  of  more  concentrated  ones ;  but  when 
the  comparison  was  made  on  the  basis,  not  of  specific,  but  of 
molecular  conductivity,  a  new  and  important  law  wras  discov- 
ered. 

For  the  purpose  of  comparing  the  molecular  conductivities 
of  solutions,  a  unit  known  as  the  "  gramme-molecule  "  was  em- 
ployed. A  given  volume  v  of  solution  is  said  to  contain  a 
"  gramme-molecule  "  of  a  given  substance  whenever  it  contains 
a  number  of  grammes  of  the  substance  equal  to  its  molecular 
\veight.  Thus  a  "  gramme-molecule "  of  potassium  cyanide 
would  be  65  grammes  supposed  to  be  dissolved  in  v  liters  of 
water.  When  v  is  one  liter  we  should  have  a  solution  of  one 
"  gramme-molecule  "  per  liter.  In  this  case,  for  univalent  sub- 
stances, the  "  gramme-molecule  per  liter  "  is  of  course  identical 
with  one  "  equivalent"  or  a  "  normal  solution." 

Xow,  when  we  compare  the  total  conducting  power  of  a 
gramme-molecule  of  all  electrolytes,  we  find  that,  as  the  volume 
v  increases,  and  the- solution  becomes  more  dilute,  the  total,  or 
molecular,  conductivity  of  the  whole  volume  of  solution  in- 


6  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

creases  instead  of  diminishing  with  dilution,  as  the  specific 
conductivity  does.  This  increase  is  at  first  quite  rapid ;  then 
the  conductivity  becomes  nearly  constant  and  increases  only 
very  slightly,  to  reach  its  maximum  value  for  v  =  infinity. 

Kohlrausch  proved  by  a  comparative  study  of  these  results 
that  the  total  conductivity  of  a  dilute  solution  is  due  to  the  ve- 
locity of  the  ions,  and  also  that  it  is  made  up  of  the  sum  of  the 
velocities  of  the  cathions  and  anions  moving  in  opposite  direc- 
tions. 

Osmotic  Pressure. — Meantime  progress  was  being  made  in  an 
apparently  totally  different  field.  Pfetfer,  professor  of  botany 
at  the  University  of  Leipzig,  made  an  extensive  study  of  the 
osmotic  transfer  of  solutions  through  the  walls  of  plant-cells, 
and  devised  in  1878  a  method  by  which  it  was  shown  that  os- 
motic action  was  capable  of  producing  certain  definite  press- 
ures. His  method  consisted  in  using  a  "  semi-pervious  mem- 
brane "  through  which  the  solvent,  but  not  the  solid  in  solu- 
tion, can  pass.  When,  for  instance,  a  glass  tube,  closed  at  the 
bottom  with  a  plug  of  porous  earthenware  coated  with  the  semi- 
pervious  membrane  of  ferrocyanide  of  copper,  is  filled  with  a 
strong  solution  of  sugar  and  the  lower  end  is  placed  in  distilled 
water,  the  latter,  being  able  to  pass  through  the  pores  of  the 
filter,  does  so;  while  the  sugar  particles,  being  unable  to  pass 
out,  remain  in  the  tube,  and  hence  the  solution  column  actually 
rises  in  the  tube.  Pfeffer  showed  that  the  pressure,  as  meas- 
ured by  the  height  of  the  column,  was  proportional  to  the 
amount  of  sugar  in  the  solution  and  increased  with  the  temper- 
ature. 

The  Gas-Law  and  Osmotic  Pressure. — These  phenomena  had 
long  been  supposed  to  be  due  to  an  attraction  of  the  sugar  for 
water;  but  the  fact  that  the  osmotic  pressure  was  proportional 
to  the  sugar-content,  and  increased  with  the  temperature,  sug- 
gested to  Prof.  Van't  Hoff,  the  brilliant  Hollander,  that  the 
dissolved  substance  acted  just  as  a  gas  would  do. 

His  reasoning  was  something  like  this  :  A  dissolved  sub- 
stance exerts  an  osmotic  pressure  against  the  bounding  surface 
of  the  liquid,  just  as  a  gas  does  against  the  walls  of  the  vessel 
that  contains  it.  But  the  surface  of  the  liquid  presses  inwards 
with  a  pressure  of  above  a  thousand  atmospheres  (the  Binnen- 
druck  of  the  Germans,  which  prevents  the  liquid  from  evapora- 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  7 

ting  instantaneously  into  space).  This  surface-tension  of  the 
liquid  resists  the  comparatively  slight  osmotic  pressure,  and  or- 
dinarily the  latter  produces  no  noticeable  effect. 

But  when  a  semi-pervious  membrane,  through  which  water 
may  pass,  but  which  will  filter  out  the  sugar  molecules,  is  in- 
terposed between  the  sugar  solution  and  the  clear  water,  the 
pressure  of  sugar  molecules  against  the  semi-pervious  mem- 
brane and  the  upper  surface  of  the  solution  raises  the  latter  just 
as  it  would  a  piston ;  and  as  the  sugar  is  unable  to  exert  any 
pressure  on  the  surface  of  the  water  on  the  other  side  of  the 
semi-pervious  membrane,  the  clear  water  freely  enters  the  tube 
through  the  membrane  as  fast  as  the  upper  surface  rises. 

Of  course,  according  to  the  gas-law,  the  osmotic  pressure 
should  increase  with  the  concentration.  Ilence  Yan't  Hoff 
applied  the  gas-law,  pv  =  RT,  in  which  p  represents  the  pres- 
sure, v  the  volume  containing  a  gramme-molecule ;  T,  the  abso- 
lute temperature ;  and  R,  the  u  gas-constant."  When  this 
formula  was  applied  to  Pfeffer's  results  an  almost  perfect  agree- 
ment was  discovered ;  and  the  same  result  was  obtained  with 
numerous  other  solutions  of  organic  substances.  But  when  it 
was  applied  to  inorganic  salts,  or  electrolytes,  it  was  found  that 
the  osmotic  pressure  was  greater  than  that  indicated  by  the 
molecular  concentration.  Yan't  Hoff  expressed  this  fact  by  the 
formula,  pv  =  iRT,  in  which  i  is  a  coefficient  greater  than 
unity. 

Here  was  an  apparent  anomaly;  the  osmotic  pressure  was 
apparently  greater  than  that  due  to  the  number  of  molecules, 
that  is,  greater  than  the  gas-law  would  indicate.  Progress 
seemed  to  be  stopped  by  a  stone  wall.  But  it  was  not  delayed 
long. 

Dissociation. — Arrhenius,  the  masterly  Swedish  physicist, 
suggested  a  new  idea.  Perhaps  the  gas-law  still  holds,  only  the 
number  of  molecules  has  been  increased  by  the  dissociation  of 
some  of  the  dissolved  substance.  If  a  part  of  the  molecules 
were  supposed  to  be  split,  so  as  to  double  their  number,  the 
total  number  of  molecules  present  would  be  increased  and  the 
gas-law  might  still  hold.  It  was  soon  shown  by  Arrhenius  that 
there  was  a  close  relation  between  the  size  of  the  coefficient  i 
and  the  "  chemical  activity  "  of  the  substance.  For  instance, 


8  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

in  the  case  of  the  inorganic  acids,  this  coefficient  was  particu- 
larly large  in  those  chemically  most  active,  like  hydrochloric 
and  nitric  acids.  Assuming  that  in  these  cases  the  dissociation 
was  most  complete,  he  made  another  brilliant  generalization. 
He  distinguished  between  "  chemically  active  "  and  "  inactive  " 
molecules,  and  claimed  that  the  dissociated  molecules  were  the 
only  "  chemically  active  "  ones.  This  distinction  has  proved 
to  be  sound. 

The  next  step  was  to  show  that  the  increase  of  molecular  con- 
ductivity of  dilute  solutions  was  due  to  this  same  dissociation, 
and  that  the  dissociated  molecules  alone  took  part  in  the  trans- 
fer of  the  electrical  current.  Arrhenius  concluded  that  the  dis- 
sociated molecules  formed  the  "ions"  which  Faraday  had 
shown  to  be  instrumental  in  conveying  the  current ;  that  each 
dissociation  produced  a  cathion  which  carried  the  positive  elec- 
trical current,  and  an  anion  which  carried  the  negative  current 
in  the  opposite  direction;  that  these  alone  were  active  in  the 
electrical  transfer;  that  when  a  substance  Avas  entirely  undis- 
sociated  it  would  be  a  non-conductor;  and  that  its  conducting 
power  wras  directly  proportional  to  the  number  of  ions  present. 

This  view  was  strongly  contested  at  first.  It  was  argued,  in 
opposition,  that  such  strongly  combined  substances  as,  for  in- 
stance, potassium  chloride,  could  not  possibly  split  up  in  solu- 
tion, even  in  part,  into  potassium  and  chlorine  ions — that  the 
potassium  wrould  decompose  the  water.  But  the  reply  was  : 
"  What  if  it  did  ?  The  only  effect  would  be  to  produce  HC1 
and  KHO  ;  these  would  again  dissociate  into  H  and  Cl  and  K 
and  HO ;  and  the  potassium  and  chlorine  ions  would  still  exist 
as  before."  It  was  also  suggested  by  Prof/Ostwald,  of  the  Uni- 
versity of  Leipzig,  that  the  ions  were  an  allotropic  modification, 
different  from  the  ordinary  elements,  in  that  to  the  cathion  was 
attached  a  positive  charge,  and  to  the  anion  an  equal  negative 
charge,  of  electricity ;  and  that  when  these  electrical  charges 
were  given  up  at  the  electrodes,  the  ions  changed  into  the  ordi- 
nary elemental  form. 

Ostwald  was  the  first  to  defend  these  new  views,  and  the  po- 
sition now  held  by  the  theory  is  largely  due  to  his  remarkable 
genius  for  outlining,  executing  and  interpreting  experimental 
work.  Together  with  his  students,  inspired  by  his  example,  he 
has  accomplished  a  great  work  in  clearing  up  many  difficult 
points  as  fast  as  they  were  raised. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


9 


Fig.  1  illustrates  an  ingenious  experiment  devised  by  Ostwald 
to  show  the  bearing  of  the  new  theory.  Two  vessels,  A  and 
B,  filled  with  a  solution  of  KC1,  are  connected  electrically  by 
the  column  of  solution  contained  in  the  siphon  C.  If  the 
theory  be  correct,  a  large  insulated  conductor  D,  charged  with 
negative  electricity,  and  brought  near  the  vessel  A,  will  act  by 
induction  on  the  ions  contained  in  A  and  B.  The  electrically 
positive,  potassium  cathions,  will  migrate  through  C  and  col- 
lect in  A,  being  attracted  by  the  negative  charge  on  D.  At 
the  same  time  the  electrically  negative  chlorine  anions,  repelled 
by  the  negative  charge  on  D,  will  accumulate  in  B. 

On  removing  C,  so  that  A  and  B  are  disconnected  electrically, 
and  then  removing  D,  the  vessel  A  will  contain  an  excess  of  posi- 


A  B 

DIAGRAM    ILLUSTRATING 

OSTWALD'S   DEMONSTRATION 

OF 
ELECTROLYTIC  DISSOCIATION 

tively  electrified  potassium  ions,  and  the  vessel  B  an  excess  of 
negatively  electrified  chlorine  ions.  So  long  as  the  vessels  are 
not  connected  with  each  other  or  the  earth,  they  will  induc- 
tively remain  in  equilibrium  and  there  will  be  no  reaction  be- 
tween them.  But  according  to  the  theory,  if  their  contents  be 
connected  by  a  platinum  wire,  the  potassium  ions  will  give  up 
their  positive  charge  on  the  end  immersed  in  A  (hydrogen  be- 
ing set  free  by  the  reaction  of  the  potassium  on  the  water  as 
soon  as  the  ions  have  given  up  their  electric  charge),  and  the 
chlorine  ions  will  give  up  their  negative  charge  on  the  end  im- 
mersed in  B  (ordinary  gaseous  chlorine  being  similarly  set  free 
on  that  end  of  the  wire). 

The  above  experiment  is  difficult  of  actual  execution  on  ac- 
count of  the  enormous  amount  of  electricity  (96,540  coulombs) 


10  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

required  by  each  gramme-molecule;  but  in  a  modified  form 
of  it,  using  a  capillary  mercurial  cathode,  Ostwald  and  Nernst 
were  able  to  demonstrate  the  correctness  of  this  view  by  traces 
of  hydrogen,  distinctly  visible  under  the  microscope. 

Fig.  1  is  therefore  not  so  much  a  record  of  actual  experi- 
mental demonstration  as  an  ideal  diagram,  serving  to  explain 
the  nature  of  the  conception  involved. 

Altered  Freezing  and  Boiling  Points. — Another  important  con- 
firmation of  the  ideas  of  Arrhenius  came  from  an  entirely  dif- 
ferent quarter.  It  had  long  been  known  that  the  boiling  point 
of  an  aqueous  solution  was  raised  and  its  freezing  point  lowered 
in  proportion  to  its  molecular  concentration;  and  the  method  had 
even  been  used  to  determine  molecular  weights  in  cases  of 
doubt.  But  here  again  dilute  solutions  proved  an  exception, 
showing  variations  in  excess  of  what  was  due,  according  to  the 
rule,  to  their  molecular  concentration.  But  when  the  behavior 
of  dilute  solutions  was  examined  in  the  light  of  the  new  theory, 
it  was  found  that  the  assumption  of  an  increase  by  dissociation 
in  the  number  of  molecules  present  explained  in  these  cases, 
also,  the  apparent  anomally.  That  is  to  say,  when  the  rise  of 
boiling  and  fall  of  freezing  points  of  dilute  solutions  were  ex- 
pressed in  terms  of  the  total  molecular  concentration  (allowing 
for  the  increase  in  number  of  molecules  by  dissociation,  as  de- 
termined by  the  method  of  electric  conductivity),  the  observed 
facts  accorded  with  the  rule. 

Heat  of  Neutralization. — Another  argument  in  favor  of  the 
dissociation  hypothesis  is  furnished  by  the  remarkable  fact  that 
the  heat  of  neutralization  of  a  gramme-molecule  of  all  dilute 
acids  is  the  same.  When  strong  solutions  of  acids  are  neutral- 
ized with  strong  solutions  of  the  several  alkalis,  the  heat  of  the 
reaction  per  gramme-molecule  is  usually  quite  different;  but 
when  dilute  solutions  are  used,  the  heat  of  neutralization  per 
gramme-molecule  is  found  to  be  practically  the  same  for  all  the 
electrolytic  salts. 

This  fact,  inexplicable  according  to  the  usual  views  of  chem- 
ical affinities,  is  a  natural  consequence  of  the  dissociation-theory. 
The  heat  of  combination  in  all  these  cases  is  practically  equal  to 
that  due  to  the  formation  of  a  gramme-molecule  of  water — that 
is,  to  the  combination  of  the  atom  of  H  in  the  acid  with  the 
molecule  of  OH  in  the  alkali.  The  0  and  OH  ions  existing  in 

S3 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  11 

water  are  infinitesimal  in  amount.  Hence,  when  two  solutions 
containing  both  in  equivalent  amounts  are  brought  into  con- 
tact, they  unite,  and  the  heat  produced  results  from  their  com- 
bination. Xo  heat  results  from  the  combination  of  the  acid 
and  alkali  radical,  because  they  were  separated  into  ions  before 
neutralization,  and  remain  in  the  same  state  afterwards. 

Hence,  as  the  heat  results  solely  from  the  reaction  H2  +  0  — 
H2O,  according  to  the  new  theory,  it  ought  to  be  the  same  for 
all  dilute  solutions  of  acids  and  alkalies. 

Solution-Pressure. — Professor  Nernst,  now  of  the  University 
of  Goettingen,  made  the  next  great  forward  step  in  explaining 
the  anomalies  in  the  electro-chemical  series.  He  investigated 
very  carefully  the  so-called  "  concentration-cells."  The  concen- 
tration-cell contains  two  electrodes  of  the  same  metal,  each  im- 
mersed in  a  solution  of  the  same  salt  of  the  metal  of  its  elec- 
trodes ;  the  only  difference  between  the  solutions  being  that  one 
has  a  greater  molecular  concentration  than  the  other.  When 
such  a  cell  is  arranged  like  a  Daniell  cell  (except  that  both 
electrodes  are,  say,  of  silver,  one  immersed  in  a  normal,  the 
other  in  a  deci-normal  solution  of  silver  nitrate),  and  the  elec- 
trodes are  connected,  a  current  of  electricity  results.  From  the 
electrode  immersed  in  the  dilute  nitrate  solution  an  equivalent 
of  silver  is  dissolved,  and  at  the  same  time  on  the  electrode  im- 
mersed in  the  concentrated  solution  an  equivalent  of  silver  is 
precipitated.  The  positive  current  flows  from  the  electrode  in 
the  weak  solution  to  that  in  the  strong  solution. 

An  electric  current  is  thus  produced  from  two  electrodes  of 
the  same  metal  immersed  in  its  own  salt.  Evidently,  therefore, 
it  can  no  longer  be  deemed  necessary  to  have  two  different 
metals,  or  even  the  salts  of  two  different  metals,  in  order  to 
produce  a  galvanic  couple.  But  whence  comes  the  electromo- 
tive force  in  this  case  ? 

An  attempt  to  answer  this  question  led  Kernst  to  propose  the 
brilliant  hypothesis  which  commonly  bears  his  name  and  rounds 
out  the  modern  theory  of  electromotive  force.  Briefly  stated,  it 
is  that,  for  a  given  temperature,  each  metal  has  a  certain  defi- 
nite "  solution-tension,"  as  he  first  called  it,  or  "  solution-press- 
ure," as  it  has  been  more  aptly  named  by  Ostwald.  -According 
to  Kernst's  idea,  every  metal  immersed  in  a  solution  con- 
taining none  of  its  ions  possesses,  at  a  given  temperature,  a  cer- 


12  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

tain  "  solution-pressure "  with  which  its  particles  tend  to  go 
into  solution  and  assume  the  ionic  state.  In  doinff  this,  each 

o 

takes  from  the  remaining  metal  a  charge  of  positive  electricity 
which  it  imparts  to  the  solution.  At  the  same  time  the  re- 
maining metal  receives  an  equal  charge  of  negative  electricity. 
This  causes  the  formation  of  Helmholtz's  "  double  layer  "  of  op- 
positely electrified  particles  on  the  surface  of  the  metal,  and  the 
process  goes  on  until  the  attraction  of  the  positively  electrified 
ions  for  the  negative  metal  just  balances  the  solution-pressure 
of  the  metal.  By  reason  of  the  enormous  electrical  charges  of 
the  atoms  (96,540  coulombs  to  the  gramme-molcule),  it  follows 
that  only  unweighable  traces  of  the  metal  have  to  go  into  solu- 
tion to  bring  about  this  equilibrium,  which,  once  assumed,  re- 
mains, unless  a  charge  of  positive  electricity  be  imparted  to  the 
negatively  electrified  metal.  When  this  is  done,  as  happens 
when  the  circuit  of  a  galvanic  battery  is  closed,  the  formation 
of  ions  and  the  solution  of  the  metal  go  on  continuously.  On 
the  other  hand,  when  a  metal  is  immersed  in  a  solution  already 
charged  with  its  own  ions,  these  at  once  set  up  an  osmotic 
pressure  opposite  to  the  solution-pressure,  and  hence,  dependent 
on  the  concentration  of  the  solution,  there  are  three  possible 
cases : 

1.  The  osmotic  pressure  of  the  ions  already  in  solution  may 
be  less  than  the  solution-pressure  of  the  metal.     Here  the  case 
is  similar  to  the  one  described  above,  but  the  difference  of  press- 
ure will  be  smaller  than  when  no  ions  were  originally  present 
in  the  solution. 

2.  The  osmotic  pressure  of  the  ions  in  solution  is  exactly 
equal  to  the  solution-pressure,  and  no  double  layer  or  difference 
of  electrical  pressure  results  between  the  metal  and  the  solution. 
In  this  case  the  metal  remains  indifferent  to  the  solution. 

3.  The  osmotic  pressure  of  the  ions   already  in  solution  is 
greater  than   the  solution-pressure  of  the  metal.     In  this  case 
the  ions  in  solution  tend  to  precipitate  themselves  on  the  sur- 
face%  of  the   metal;  and  at  the  same   time  they  impart  their 
charge  of  positive  electricity  to  the  metal,  which  becomes  posi- 
tively electrified,  while  the  solution  which  had  contained  an 
equal  number  of  positive  and  negative  ions  becomes  negatively 
electrified.     This  soon  produces  a  new  double  layer  of  oppo- 
sitely electrified  ions,  which  brings  about  an  equilibrium,  unless 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  13 

a  continuous  stream  of  negative  electricity  is  imparted  to  the 
positively  electrified  metal  (as  by  completing  the  circuit  of  a 
galvanic  battery),  when  the  precipitation  of  the  ions  goes  on 
continuously.  The  equilibrium  of  the  double  layer  is  brought 
about  (for  the  same  reason  as  given  before)  by  the  precipitation 
of  unweighable  traces  of  the  ions. 

According  to  this  view,  the  electromotive  force  of  a  galvanic 
cell  is  just  as  much  due  to  the  difference  of  pressure  between 
the  metals  tending  to  assume  the  ionic  state,  and  the  ions  tend- 
ing to  assume  the  metallic  state,  as  the  force  of  a  steam  or  com- 
pressed-air engine  is  due  to  the  differences  between  the  steam- 
or  air-pressures  acting  on  either  side  of  the  piston. 

Hence,  Xernst  applied  the  laws  of  thermodynamics,  which  had 
been  already  worked  out  so  thoroughly  for  gases,  to  this  prob- 
lem also,  and  with  the  most  remarkable  results.  It  is  impossible 
to  enter  here  into  all  the  refinements  of  the  subject;  but  the 
following  condensed  statement  will  give  an  idea  of  the  reason- 
ing involved. 

Starting  with  the  well-known  gas-law 

(1)  pv  =  ET. 

(p  and  P  being  pressures  in  grammes  per  square  centimeter;  Y 
and  r,  the  corresponding  volumes  in  ccm.  to  contain  one  gramme- 
molecule  ;  T,  273  4-  t  degrees  Centigrade ;  and  R,  the  "  gas- 
constant"  =  1.96  calories),  .we  can  easily  determine  the  maxi- 
mum amount  of  work,  A,  in  gramme-centimeters,  done  by  a 
perfect  gas,  expanding  at  a  constant  temperature  from  a  volume 
v  to  a  larger  volume  V,  and  at  the  same  time  falling  from  a 
pressure  P  to  a  smaller  pressure  p.  We  have : 

P 

(2)  A  =  Ctdp. 

PJ 

But  from  (1),  v  =    5T.  hence 

P 

(3)  A  ==BTfdp 

p    y 
and,  integrating  this,  we  have 

(4)  A  =  RT  nat.  log.  -• 

P 


14  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

If  we  now  suppose  the  gas-law  to  apply  to  the  diffusion  of  the 
ions  from  the  electrode  into  the  solution  ;  let  P  represent  the  "  so- 
lution-pressure "  and  p  the  "  osmotic  pressure  "  of  the  ions  of  the 
given  metal  in  solution;  and  further  assume  that  work  is  done 
only  in  falling  from  the  pressure  P  to  the  lower  osmotic  pressure 
p,  and  not  in  changing  from  a  solution-pressure  P  to  an  equal 
osmotic  pressure  P,  it  follows  that  equation  (4)  will  apply  di- 
rectly to  this  case  also. 

But  we  can  also  express  the  maximum  work  A  in  electrical 
units.  If  we  assume  that  to  a  "  gramme  equivalent  "  of  a  uni- 
valent  element  is  given  its  unit  charge  of  e  =  96,540  coulombs 
of  electricity,  and  represent  by  the  Greek  letter  TT  the  potential 
difference  in  volts  between  the  metal  and  the  solution,  we  may 
also  represent  the  work  A  done,  by  the  product  *  e.  Hence  we 
have  : 

(5)  TT  e  =  RT  uat.  log.  -  ; 

p 

or,  changing,  for  convenience  of  calculation,  from  natural  to 
common  logarithms,  we  have 

RT  P 

(6)  x  e  =  -    —  lo^.  —  .    and  hence 
V  0.4843       '   / 

RT  P 


e  X  0.4343       '  p 

Now  R  =  1.96  calories,  or  in  electrical  units,  R  =  1.96  X  4.24  ; 
e—  96,540  coulombs;  and  for  ordinary  temperatures  t  —  17° 
C.  or  T  =  273  +  17  =  290°  C.  Substituting  these  values,  we 
have  : 

1.96X4.24X290,       P      AARf?R1       P      u 

(8)  TT  =  —  log.  —  =  0.0575  log.  _  volts. 

96,540  X  0.4343  p  5  p 

For  ions  that  have  a  valency  n  >  1,  each  gramme-molecule  will 
require  n  X  e,  or  n  X  96,540  electrical  units,  and  this  must  be 
substituted  in  the  formulas  (5)  to  (8)  instead  of  e  ;  when  this 
is  done  we  have  the  general  formula  : 

0.0575,       P      u 

(9)  TT  =  _    -  W.  ~  volts. 

n  p 

This  formula,  of  course,  is  only  true  for  t=.  17°  C. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  15 

I  have  plotted  this  curve  Mn  Fig.  2.     It  will  serve  for  any 

p 
metal    in  which   the   ratio  --  is  the  same.     If  we  studv  the 

P 

p 
ratios  of  -     we  see  that  when  P  is  greater  than  p,  TT  always 

P 

has  a  positive  value ;  that  is,  the  liquid  is  positively  electrified 
by  the  ions  which  go  into  solution,  and  the  remaining  mass  of 

metal  is,  in  consequence,  negatively  electrified.     As  a  further 

p  p 

consequence,  when  p  =  0,  _  =  infinity;  hencelog.  -,  and  hence 

TT,  equals  infinity. 

This,  of  course,  is  interpreted  to  mean  that  a  metal  brought 
into  the  presence  of  a  solution  containing  none  of  its  ions  would 
have  an  infinite  potential  writh  regard  to  that  solution ;  but  this 
could  only  last  for  an  infinitesimal  period,  after  which  the  liquid 
would  be  impregnated  with  the  ions  of  the  metal.  Experiment 
shows  that  none  of  the  metals  give  an  infinite  potential  in  any 
known  solution.  It  follows,  therefore,  that  traces  of  the  ions 
of  all  the  metals  must  exist  in  all  solutions,  even  though  they 
may  not  be  recognizable  by  any  other  chemical  or  physical 
test.  The  same  mathematical  difficulty  exists  with  regard  to 
the  conception  of  a  perfect  vacuum,  and  a  similar  conclusion 
may  be  drawn,  namely,  tha,t  such  a  thing  as  a  perfect  vacuum 
is  physically  impossible. 

P  P 

When  P  =  p,  then  _  =  1,  and  log.  _  =  0.     In  this  case,  the 

potential  is  0,  and  there  is  no  tendency  either  to  dissolve  or  to 

precipitate  the  metal. 

p 
When  P  is  less  than  p,  log.  -  is  negative,  and  the   solution 

P 

is  negatively  electrified,  owing  to  the  positive  ions  precipitating 
themselves  with  their  positive  charges  upon  the  metal,  which 

becomes  positively  electrified.      If  p  could  become  infinity, 

p 
log.  —  would  become  minus  infinity.     As  a  matter  of  fact,  these 

values  are  never  reached,  for  the  simple  reason  that  as  p  de- 
pends on  the  number  of  ions  in  a  unit-volume,  it  follows  that, 
on  dilution,  p  reaches  nearly  a  maximum  value  for  very  moder- 
ate dilutions,  when  dissociation  of  the  liquid  is  nearly  complete ; 
and,  after  that,  the  value  of  p  is  reduced  rather  than  increased 
by  further  dilution. 


16  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

According  to  this  view,  the  electromotive  force  of  the  metals 
in  their  solutions  depends,  for  a  given  temperature : 

1.  Upon  the  "  solution-pressure  "  peculiar  to  each  metal. 

2.  Upon  the  number  of  ions  of  the  given  metal  present  in 
the  solution  in  which  it  is  immersed. 

If  the  "  solution-pressure "  peculiar  to  each  metal  were 
accurately  known,  we  might  write  out  an  absolute  electro- 
motive series  for  the  metals.  Le  Blanc*  proposes,  on  the 
basis  of  the  work  of  Neumann,  and  on  the  supposition  that  the 
osmotic  pressure  of  a  totally  dissociated  normal  solution  (con- 
taining one  gramme-molecule  in  the  dissociated  state)  is  equal 
to  22  atmospheres,  a  series  of  this  kind,  as  follows : 

Electromotive  Series  of  Metals  in  Solution. 

(Value  of  P  at  17°  C. ) 


Atmospheres. 

Atmospheres. 

18 

—3 

Zinc, 

.     9.9  X  10 

Lead, 

.   1.1x10 

6 

—4 

Cadmium,  . 

.     2.7  X  1U 

Hydrogen, 

.     9.9.X  10 

2 

-20  (?) 

Thallium,  . 

.     7.7X10 

Copper, 

.     4.8X10 

4 

—16 

Iron,  . 

.     1.2X10 

Mercury,   . 

.   1.1x10 

0 

—17 

Cobalt,       . 

.     1.9X10 

Silver, 

.     2.3X10 

0 

-36 

Nickel,       . 

.     1.3X10 

Palladium, 

.     1.5X10 

But  it  is  in  the  varying  number  of  ions  present  in  solution 
that  the  true  explanation  of  the  apparent  anomalies  in  the  elec- 
tromotive series  was  found.  According  to  this  theory  the  posi- 
tion of  the  metal  in  the  series  ought  to  vary  in  different  solu- 
tions in  accordance  with  the  number  of  ions  of  the  given  metal 
that  can  exist  in  the  given  solution. 

Complex  Ions. — The  anomalous  position  of  copper,  gold  and 
silver  in  cyanide  solutions  is  here  explained  for  the  first  time. 
According  to  this  view,  there  are  very  few  metallic  ions  of 
these  metals  in  solutions  of  their  cyanides.  For  example,  the 
double  cyanide  of  gold  and  potassium  (potassium  auro-cyanide) 
dissociates,  in  part,  first  into  a  positive  ion  K  (+)f  and  a  nega- 

*  Elements  of  Electro-chemistry,  p.  228. 

t  The  expression  (  +  )  means  that  the  ion  after  which  it  is  written  carries  a 
positive  charge  of  electricity ;  the  expression  (— )  means  that  the  ion  carries  a 
negative  charge,  and  (±)  that  it  is  neutral,  or  not  electrified. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


17 


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o    rf   =>•   <=   o-   c5   o   =>   o    o  =5   «   « 
-l-++-t-  +  +++++-t- 


=-0  "o  'o  %  "o   o  «s  -g  2o  §o  3o  S> 
X 


>  °  S  2  §  o-  5  3  S  S  3  5  5  § 
I    I  I   I   I  1   I   I   I  I   I   I 


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CD 

. 


L  _L  I  J_ 


LJ 


0>  O  T- 

0  ^  ^ 


live  ion  AuCy2  ( — ).  The  latter,  in  accordance  with  the  "  mass- 
law,"  also  dissociates,  to  a  very  slight  extent,  into  AuCy  (±)  and 
Cy  ( — )  and  the  AuCy,  in  accordance  with  the  same  mass-law, 
dissociates,  to  an  almost  infinitesimal  extent,  into  Au  (4-)  and 

2 


18  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

Cy  ( — ).  The  metallic  gold  ions,  thus  existing  to  an  almost  in- 
finitesimal extent  in  cyanide  solutions,  are  the  only  ones  that 
exert  an  osmotic  pressure  against  the  solution-pressure  of  the 
gold.  Hence,  in  spite  of  the  low  solution-pressure  of  the  gold, 
the  still  lower  osmotic  pressure  of  the  few  gold  ions  present 
renders  the  potential  of  the  gold  in  cyanide  solutions  remark- 
ahly  high.  Its  solubility,  also,  is  thereby  explained. 

But  a  high  potential  difference  does  not  necessarily  indicate 
the  great  solubility  of  a  metal ;  it  may,  in  fact,  indicate  the  op- 
posite. For  instance,  the  electromotive  force  of  silver  in 
cyanide  of  potassium  solutions  is  high ;  but  in  sulphide  of  po- 
tassium solutions  it  is  still  higher — owing,  in  this  case,  to  the 
extreme  insolubility  of  the  sulphide  of  silver.  This  extreme 
insolubility  of  the  sulphide  of  silver  reduces  to  a  minimum  the 
number  of  metallic  silver  ions  that  are  present  in  the  solution, 
diminishes  the  osmotic  pressure  of  the  ions,  and  hence  in- 
creases the  electromotive  force. 

The  explanation  of  these  remarkable  exceptions  that  "prove 
the  rule,"  is  due  to  the  work  of  Ostwald,  who,  more  than  any 
one  else,  has  filled  in  the  gaps  and  explained  away  the  difficul- 
ties presented  by  the  new  views. 

Beyond  doubt,  the  gold,  the  silver  and  the  copper  in  the 
cyanide  solution  are  mainly  combined  with  Cy2  to  form  electro- 
negative ions  AuCy2( — ),  AgCy2( — ),  and  CuCy2( — ).  As  to 
silver,  this  conclusion  is  to  be  drawn,  in  fact,  from  Hittorf  s 
early  experiments,  and,  as  to  gold  and  copper,  from  those  of 
Ostwald,  already  described.*  These  ions  have  been  termed  by 
Ostwald  "complex  ions,"  to  indicate  that  they  contain  the 
metals  in  a  combination  in  which  their  ordinary  chemical  re- 
actions are  entirely  masked.  There  are  many  other  such 
combinations :  the  thiosulphites  of  gold  and  silver,  the  ferro- 
cyanides  and  ferricyanides,  the  platinochlorides,  etc.,  all  fail  to 
answer  the  ordinary  tests  for  the  gold,  silver,  iron,  and  plati- 
num that  they  contain.  The  alkaline  sulphhydrates  of  many 
of  the  metals  are  also  examples  of  the  same  fact. 

It  is  for  this  reason  that  all  of  the  ordinary  reagents  fail  to 
precipitate  the  gold  and  silver  from  cyanide  solutions.  For 

*  See  Christy,  "  The  Solution  and  Precipitation  of  the  Cyanide  of  Gold,"  T,ans  , 
xx  vi.,  758  et  seq. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  19 

this  reason,  also,  the  electric  current  causes  a  transfer  of  the 
electronegative  ion  from  the  negative  to  the  positive  pole,  or 
anode,  in  the  opposite  direction  to  that  ordinarily  taken  by  the 
metals,  and  only  the  comparatively  few  electropositive  gold 
ions  present  in  the  solution  travel  towards  the  cathode  or  nega- 
tive electrode  of  the  deposition-box.  As  already  shown  in  my 
paper,  just  cited,  this  necessarily  retards  the  electrodeposition 
of  the  gold  from  cyanide  solutions.  However,  the  potassium 
ions,  on  giving  up  their  electric  charge  to  the  cathode,  pre- 
cipitate gold  from  the  adjacent  solution,  and  this  helps  matters 
out.  Nevertheless,  the  travel  of  the  AuCy2( — )  ions  to  the 
anode  considerably  retards  the  precipitation  of  the  gold. 

To  illustrate  the  great  difference  in  the  osmotic  pressure  ex- 
erted by  the  complex  ions  as  compared  with  the  ordinary  case 
of  dissociation,  the  following  examples  are  cited  from  the  work 
of  Ostwald  and  his  pupils  : 

Ordinary  dissociation  is  represented  by  the  cases  of  solutions 
of  potassium  chloride,  copper  sulphate,  silver  nitrate,  and  hy- 
drochloric acid.  These  are  found  to  be  dissociated  as  follows : 

KCl  =  K(  -f)  +C1(— )     Practically  wholly  dissociated  at      M 


CuSO4  =  Cu(+ )  +  SO4  (— )     Practically  wholly  dissociated  at 


10,000 
M 


10,000 

AgNO3  =  Ag(  -f)  +NO3  (— )  Practically  wholly  dissociated  at  - 

1,000 

HC1  =  H(-f)  +C1  (— )     Practically  wholly  dissociated  at  — 

100 

The  above  characteristic  cases  show  that  the  degree  of  dis- 
sociation varies  extremely  with  different  salts,  but  with  many 
substances,  like  silver  nitrate  and  hydrochloric  acid,  is  practi- 
cally complete  at  very  moderate  dilutions. 

Let  us  take  in  contrast  the  case  of  a  complex  ion,  that  pro- 
duced, for  instance,  by  the  dissociation  of  potassium  argento- 
cyanide.  According  to  an  investigation  of  Morgan,*, the  dis- 
sociation takes  place  in  three  steps.  Of  these;  the  first  is  very 
complete  : 

KAgCy,  (±)  =  K(+)  +  AgCy2  (-). 

(The  latter  is  the  "  complex  ion.") 

'  *  Zeitsch.filr  Phys.  Chemie,  Bd.  xvii.,  S.  513. 


20  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

The  second  occurs  in  very  small  amount : 

AgCy2(-)  =  AgCy(±)  +  Cy(-). 
The  third  is  in  almost  infinitesimal  traces : 
AgCy(±)=Ag(+)  +  Cy(— ). 


M 

In  the  case  of  —  KAgCy2,    Morgan    shows    that   the    first 
20 

-3 

step  is  almost  complete;  the  second  step  is  2.76  X  10  M  =  5 
per  cent,  dissociated;  and  the  third  step  shows  of  Ag  (-f)ions, 

3.65X10  M  =  3.65  X  108  X  1000  X  10  =3.94x10  mg. 
per  liter; — that  is,  there  are  only  about  four  millionths  of 
a  milligramme  of  silver  in  the  ionic  state  in  a  liter  of  such  a 

solution. 

M 

Let  us  compare  this  with  the  dissociation  in  a  —  AgN03 

solution.  Morgan  finds  this  to  be  86.5  per  cent,  dissociated, 
hence  a  liter  of  such  solution  will  contain  0.865  X  108  X  - 

3 

=  4.36  X  10  mg.  of  Ag(-f)  ions  per  liter. 

The  ratio  of  silver  ions  in  the  cyanide  solution  to  those  in 
the  nitrate  solution  is  therefore : 

4.36  X  103    _  1  n      1Q9> 
3.94xlO-6~ 

Hence,  there  are  over  a  billion  times  as  many  silver  ions  in 

_  silver  nitrate  as  in  silver  cyanide. 
20 

This  makes  clear  at  once  the  reason  of  the  great  difference 
between  the  osmotic  pressure  of  the  silver  ions  in  the  nitrate 
and  in  the  cyanide  solution,  and  consequently  the  reason  why 
the  electromotive  force  of  silver  is  so  much  greater  in  the 
cyanide  solutions.  The  case  of  potassium  aurocyanide  and 
other  complex  salts  is  entirely  similar  to  that  of  potassium 
argento-cyanide.  It  should  be  further  stated  that,  according 
to  this  view,  ions  can  only  form  or  disappear  in  infinitesimal 
traces,  sufficient  to  bring  about  a  static  equilibrium,  unless 
they  appear  or  disappear  in  pairs,  positive  and  negative,  as  they 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  21 

do  in  the  continuous  passage  of  the  current  through  the  elec- 
trolytic cell.  It  is  for  this  reason  that  it  is  necessary  to  have 
two  electrodes  to  any  galvanic  cell,  one  to  receive  the  discharge 
of  electricity  from  the  positively  electrified  cathions,  and  the 
other  to  receive  that  from  the  negatively  electrified  anions. 
Without  both  of  these,  a  continuous  current  is  impossible. 

In  considering  this  case  of  the  Daniell  cell,  for  instance  : 
If  we  represent  the  solution-pressure  of  the  zinc  by  Pv  the  os- 
motic pressure  of  the  zinc  ions  present  in  the  zinc  sulphate 
by  pl9  and  the  similar  values  for  the  copper  by  P2  and  for  cop- 
per ions  in  the  copper  sulphate  by  pv  we  shall  have  for  the 
electromotive  force  of  the  zinc  in  zinc  sulphate  : 


and  for  the  copper  in  copper  sulphate, 

0.0575 


Either  of  these  alone  can  give,  not  a  continuous  current,  but 
only  a  static  charge  of  ions,  which  prevents  further  action  ; 
but  when  they  are  combined  on  a  closed  circuit,  as  in  the 
Daniell  or  gravity-cell,  we  have  a  resulting  difference  of  poten- 

tial: 

0.0575 


0.0575  -       P.  X  p, 
~2"     g'  ^XF,' 

On  the  other  hand,  in  a  concentration-cell,  with  either  ot 
these  metals  (copper  electrodes,  for  instance),  one  in  a  strong 
and  the  other  in  a  weak  solution  of  copper  sulphate,  we  should 
have  Px  =  P2  in  the  above  formula,  and  the  only  difference 
would  be  in  the  differing  concentration  of  the  ions  pl  and  p2  in 
the  strong  and  weak  solutions.  Making  these  substitutions  in 
the  formula,  we  should  have,  in  this  case  : 


22  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

In  this  case  it  is  evident  that  the  solution-pressure  of  the  metal 
itself  plays  no  part  in  creating  the  electromotive  force  "of  the 
combination.  It  is  entirely  due  to  the  relative  number  of  ions 
present  in  the  strong  and  weak  solutions.  The  solution  con- 
taining the  smaller  number  of  ions  per  unit-volume  will  oppose 
to  the  solution-pressure  of  the  metal  immersed  in  it  the  smaller 
resistance,  and  that  metal  will  dissolve  and  the  other  will  pre- 
cipitate. 

The  above  outline  gives  only  in  the  briefest  way  a  summary 
of  some  of  the  hardest  thinking  and  closest  reasoning  that  has 
been  done  of  late  in  one  of  the  most  important  departments  of 
physical  chemistry.  It  is,  perhaps,  too  much  to  hope  that  the 
conclusions  reached  are  all  final.  But  they  rest  on  such  a  firm 
foundation  of  experimental  work,  and  explain  so  much  that  is 
otherwise  inexplicable,  that  it  is  certain  that  they  contain  a 
large  amount  of  truth.  The  details  will  probably  be  filled  in 
quite  differently  from  what  we  now  expect,  but  the  great  mass 
seems  to  be  solid  gain.  The  importance  of  these  new  views  in 
the  study  of  chemical  geology  and  the  deposition  of  ores,  in 
every  branch  of  the  metallurgical  art  and  in  all  departments  of 
practical  chemistry,  has  hardly  begun  to  be  appreciated  and 
utilized.  It  is  not  too  much  to  say  that  ionic  action  lies  at  the 
bottom  of  all  plant-  and  animal-life  ;  and  that  the  new  views 
are  sure  to  create  a  revolution  in  the  study  of  physiological 
chemistry,  biology,  pathology  and  therapeutics.  In  every  de- 
partment of  physical  science  where  they  have  been  applied, 
they  have  acted  like  a  new  ferment ;  this  speaks  volumes  for 
their  usefulness  and  virility.* 

Objections  to  the  New  Electrolytic  Theory. — The  above  theories 
have  not  been  received  without  opposition  from  many  chemists 
and  physicists  of  no  little  weight.  Many  of  the  first  opponents 
to  the  theory  have  been  overthrown,  and  point  by  point  has 

*  Those  who  are  interested  in  following  up  these  ideas  more  at  length  will  find 
the  subject  treated  in  extenso  in  Ostwald's  Elektrochemie,  ihre  Geschichte  und  Lehre, 
Leipzig,  1896  (1150  pp.),  his  Chemische  Energie,  Leipzig,  1893  (1090  pp.),  and 
Nernst's  Theoretische  Chemie,  Stuttgart,  1893  (580  pp.).  An  admirable  sumrnnry 
of  these  views  appears  in  The  Elements  of  Electro-chemistry,  translated  into  English 
by  W.  R.  Whitney,  from  the  German  of  Le  Blanc.  Macmillan  &  Co.,  London 
and  New  York,  1896  (pp.  282).  Since  this  paper  was  presented,  another  excel- 
lent work  has  appeared  :  Theory  of  Electrolytic  Dissociation,  by  H.  C.  Jones.  Mac- 
millan &  Co.,  1900,  pp.283. 


THE    ELECTROMOTIVE    FORCE    OF   METALS.  23 

been  won  against  the  strongest  opposition.  But  the  field  is  by 
no  means  clear  of  weighty  objectors.  The  English  school,  led 
by  Pickering  with  his  "  hydrate  theory,"  has  opposed  most  bit- 
terly the  new  theory  of  "  dissociation.  Others,  such  as  Cromp- 
ton  in  England,  and  Bucherer  in  Germany,  have  proposed 
what  may  be  termed  the  "  association  "  as  opposed  to  the  "  disso- 
ciation "  theory.  The  battle  still  rages.  While  the  issue  seems 
certain  in  the  main  to  be  in  favor  of  the  new  views,  there  is  not 
wanting  evidence  of  the  wisdom  of  a  compromise  on  certain 
minor  but  important  points. 

It  will  be  noticed  that  the  adherents  of  the  dissociation 
theory  neglect  in  toto  the  effect  of  the  solvent.  Formerly, 
chemists  fixed  their  eyes  on  the  mystic  power  of  the  solvent  to 
clear  up  any  doubtful  question.  The  new  theorists  ignore  the 
solvent  entirely.  It  is  indeed  astonishing  what  they  have  been 
able  to  explain  without  it,  but,  like  Banquo's  ghost,  "  it  will 
not  down."  They  have  assumed  the  solvent  to  be  without 
action  in  bringing  about  dissociation  and  electrolysis.  They 
have  regarded  it  simply  as  of  the  nature  of  a  vacuum  into 
which  the  ions  were  free  to  expand,  and  everything  has  been 
supposed  to  be  due  to  the  pressure  of  the  ions,  nothing  to  the 
medium. 

But  already  there  are  signs  of  reaction.  It  has  been  noticed 
that  not  all  solvents  are  capable  of  changing  salts  into  electro- 
lytes. Some  are  almost  without  effect  in  this  respect.  In  other 
words,  not  all  liquids  are  capable  of  becoming  vacua  into 
which  the  ions  may  evaporate.  This  fact  of  itself  is  enough  to 
show  that  the  nature  of  the  solvent  is  not  without  influence  on 
the  dissociation. 

It  was  next  noticed  that  most  of  the  solvents  which  enable 
electrolysis  to  take  place  contain  oxygen,  and  that  of  these, 
those  possess  the  power  most  strongly  which  contain  the  most 
oxygen,  ^ext,  it  was  suggested  that  in  all  probability  oxygen 
is  quadrivalent  rather  than  bivalent,  as  usually  supposed.  The 
fact  that  carbon,  which  is  never  known  to  be  anything  but 
quadrivalent,  combines  with  oxygen  to  form  carbon  monoxide 
(CO),  favors  this  view.  If  oxygen  be  regarded  as  at  least  po- 
tentially quadrivalent,  an  explanation  is  at  once  found  for  the 
well-known  variations  in  the  water  of  crystallization  of  salts; 
for  Pickering's  remarkable  series  of  u  hydrates;"  and  last,  but 


24  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

not  least,  the  idea  suggests  itself:  "  May  not  the  ions  be,  not 
merely  a  dissociation  of  the  dissolved  salt,  but  an  association  of 
the  dissociated  ions  with  one  or  more  water  molecules  ?" 

It  was  first  supposed  that  all  solvents  capable  of  forming  elec- 
trolytes contained  oxygen.  This  was  disproved  by  the  dis- 
covery cited  by  Ostwald,  that  liquefied  ammonia  (NH3),  a  non- 
conductor, becomes  a  conductor  when  salts  are  dissolved  in  it. 
This  was  cited  to  prove  that  the  ionizing  force  was  not  due  to 
the  presence  of  oxygen.  But  it  was  pointed  out  by  Bruehl,*  that, 
like  oxygen,  the  nitrogen  in  NH3  has  two  unsatisfied  valencies, 
which  are  thus  capable  of  acting  like  it  in  producing  ionization. 
He  predicts  that  anhydrous  HCN  when  liquefied,  as  well  as  PC13 
and  AsCl3,  will  be  likely  to  have  similar  effects  for  similar  rea- 
sons. Nernstf  has  also  called  attention  to  the  sigular  propor- 
tionality between  the  dissociating  power  of  solvents  and  their 
dielectric  constant.  The  latter  are  as  follows  for  certain  sol- 
vents : 

Dielectric  Constants  (Nernst). 


GflSeS    . 

1  00 

Hydrocarbons, 

1.7  to  2.6 

CS-2 

.         .         .           2.6 

Ether, 

4.1 

Esters, 

6.9   . 

Acetic  Acid, 

9.7 

Alcohol, 

/                                  26.0 

Water, 

bO.OO 

This  series  might  almost  serve  as  showing  the  relative  disso- 
ciating-power  of  these  substances. 

ThuringJ  had  also  called  attention  to  the  remarkable  differ- 
ence of  the  dielectric  constants  between  water  and  ice,  that  of 
water  at  0°  C.  being  79.46,  and  that  of  ice  at-2°  C.  being 
3.36  only.  He  also  gives  10.30  as  the  constant  for  liquid  acetic 
acid,  and  2.79  for  solid.  In  all  these  cases,  the  dissociating 
power  increased  with  the  dielectric  constant.  This  points  to 
relations  worth  following  to  a  conclusion. 

It  does  seem,  then,  as  if  the  adherents  of  the  dissociation 
theory  had  ignored  too  much  the  effect  of  the  solvent,  and  as  if 
the  final  theory  must  be  enlarged  to  include  it,  Why  should 

*  Z.f.  Phys.  Ch.,  xxvii.,  319,  (1893).         f  Z.  f.  Phys.  Ch.,  xiv.,  622  (1894). 
J  Z.f.  Phys.  Ch.,  xiv.,  286  (1894). 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  25 

the  action  of  the  solvent  be  ignored  ?  Its  chemical  action,  it  is 
true,  is  often  slight,  but  when  we  take,  into  account  the  effect 
(in  dilute  solutions),  of  its  relatively  great  mass,  does  not  this 
explain  why  dilute  solutions  are  more  dissociated  than  strong 
ones  ?  In  the  latter,  the  cohesion  of  the  solid  still  maintains, 
even  in  solution,  a  certain  effect;  but  as  the  mass  of  the  salt 
diminishes,  and  that  of  the  solvent  increases,  the  latter  makes 
up  by  its  mass  what  it  lacks  in  intensity. 

If  the  solvent  is  without  effect,  why  does  not  the  solution- 
pressure  of  the  metals  cause  them  to  ionize  as  freely  into  a 
vacuum,  or  into  the  air  ?  And  why  should  one  solvent  be  ef- 
fective and  not  another  ? 

The  dissociation-controversy  in  England  waxed  particularly 
warm  in  the  numbers  of  Nature  published  in  1897.* 

One  can  hardly  read  this  discussion  without  feeling  that,  on 
the  whole,  the  dissociation  theory  has  the  better  of  it.  But  the 
associationists,  led  by  Pickering,  are  not  wholly  wrong.  Pick- 
ering cites  one  experiment  that  is  worth  quoting.  He  says  that 
when  a  solution  of  propyl-alcohol  in  water  is  placed  within  a 
semi-permeable  membrane,  the  water  from  without  passes 
through  the  membrane  into  the  interior.  This  seems  to  prove 
that  the  membrane  is  pervious  to  water  but  not  to  propyl-alco- 
hol. But,  he  adds,  when  the  same  vessel  is  immersed  in  pro- 
pyl-alcohol the  propyl-alcohol  passes  in  through  the  membrane, 
but  the  water  cannot  get  out.  This  would  seem  to  prove  that 
the  membrane  was  pervious  to  the  propyl-alcohol  but  not  to  the 
water.  He  considers  this  a  reductio  ad  absurdum.  But  Wetham 
points  out  that  the  experiment  may  be  interpreted  to  mean  that 
the  membrane  is  pervious  to  either  water  or  to  propyl-alcohol, 
but  not  to  their  associated  molecules  or  to  the  solution  of  one 
in  the  other. 

Wetham  also  points  out  that  the  assumption  that  the  ions  are 
dissociated  from  each  other  does  not  in  any  way  contradict  the 
assumption  that  they  are  severally  associated  in  some,  as  yet 
unkown,  manner  with  the  solvent. 

*  Nature,  vol.  lv.,  Dr.  H.  E.  Armstrong,  p.  78,  against ;  Prof.  O.  J.  Lodge,  p. 
151,  for ;  W.  C.  D.  Wetham,  for,  p.  152 ;  Spencer  Pickering,  against,  p.  22  *  ; 
Lord  Raleigh,  p.  2o8,  for;  Lord  Kelvin,  p.  273,  agnostic,  if  not  wholly  skep- 
tical ;  Prof.  J.  Willard  Gibbes,  p.  461,  for,  answers  some  of  Lord  Kelvin's  objec- 
tions ;  W.  C.  D.  Wetham,  p.  606,  for,  answers  Pickering.  The  discussion  is  con- 
tinued in  Nature,  vol.  Ivi.,  p.  29. 


26  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

Lord  Kelvin  (loc.  tit.,  p.  273)  takes  an  agnostic,  if  not  skepti- 
cal, position.     After  stating  the  problem  of  osmotic  pressure,  he 


"  No  molecular  theory  can,  for  sugar  or  common  salt  or  alcohol  dissolved  in 
water,  tell  us  what  is  the  true  osmotic  pressure  against  a  membrane  permeable  to 
water  only,  without  taking  into  account  laws,  quite  unknown  to  us  at  present,  re- 
garding the  three  sets  of  mutual  attractions  or  repulsions  :  (1)  between  the  mole- 
cules of  dissolved  substance  ;  (2)  between  the  molecules  of  the  water  ;  (3)  between 
the  molecules  of  the  dissolved  substance  and  the  molecules  of  the  water." 

He  follows  this  with  a  warning  against  undue  haste  in  ac- 
cepting theoretical  views  as  settled  while  they  are  still  open  to 
debate. 

On  p.  461  (loc.  tit.}  Prof.  J.  Willard  Gibbes  shows  that  in  the 
case  cited  by  Lord  Kelvin,  for  dilute  solutions,  where  the  rela- 
tion of  the  density  and  pressure  of  the  dissolved  substance  be- 
comes like  that  of  a  gas,  it  is  only  necessary  to  have  a  single 
numerical  constant  in  addition  to  the  relation  between  the  den- 
sity and  the  osmotic  pressure  to  solve  the  problem. 

It  must  be  remembered  that  the  greatest  triumphs  of  the 
new  theory  are  confined  to  dilute  solutions,  but  a  complete 
theory  of  solutions  must,  of  course,  include  all  states  from  the 
dilute  solution  through  the  saturated  solution  to  the  solid  sub- 
stance with  its  various  hydrates. 

In  describing  the  condition  of  affairs  assumed  in  the  new 
theory  of  ionic  dissociation,  Le  Blanc  says  :* 

"  The  parts  resulting  from  the  dissociation  (the  ions)  are  electrically  charged, 
and  contain  equivalent  amounts  of  positive  and  negative  electricity.  It  is  natural 
to  ask  :  Whence  come  these  sudden  charges  of  electricity?  They  seem  to  be  pro- 
duced from  nothing.  An  answer  that  seems  satisfactory  is  not  difficult  to  give. 
It  is  known  that  metallic  potassium  and  iodine  combine  to  form  potassium  iodide. 
In  this  combination  heat  is  generated,  which  shows  that  the  two  have  entered 
into  a  state  in  which  they  contain  less  energy  than  before.  A  certain  amount  of 
chemical  energy  doubtless  still  remains  in  the  compound,  and  when  the  salt  is 
dissolved  in  water,  the  greater  part  of  this  chemical  energy  is  changed  into  elec- 
trical, through  the  influence  of  the  solvent.^  This  energy  is  seated  in  the  charges  of 
the  ions.  The  potassium  ion  is  positively,  and  the  iodine  negatively  electric. 
By  the  aid  of  the  electric  current,  it  is  possible  to  add  to  these  ions  the  energy  in 
the  form  of  electricity  necessary  to  give  them  the  energy  they  originally  pos- 
sessed as  elements.  In  such  a  case,  they  separate  in  the  ordinary  molecular 
forms  at  the  electrodes." 


*  Le  Blanc,  Elements  of  Electro-chemistry,  p.  60. 
t  The  italics  are  mine.— S.  B.  C. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  27 

It  will  be  noticed  that  in  this  explanation  of  the  mode  of  for- 
mation of  the  ions  on  the  dissociation  hypothesis,  Le  Blanc 
uses  the  phrase  :  "  through  the  influence  of  the  solvent."  That  is, 
he  seems  to  recognize  the  need  of  bringing  this  influence  into 
the  problem.  He  does  not  attempt  to  show  how  it  acts.  But 
if  it  is  able  to  alter  such  strong  affinities  as  those  of  potassium 
and  chlorine,  or  even  of  potassium  and  iodine,  by  changing  the 
chemical  into  electric  energy,  such  action  is  surely  worth 
studying  most  closely. 

I  am  firmly  convinced  that  the  next  great  advance  will  be 
made  when  the  effect  of  the  solvent  is  more  closely  studied. 
But  while  believing  that  the  association  or  loose  combination 
of  the  water-molecules  with  the  dissociated  ions  plays  an  im- 
portant, though  as  yet  unknown,  part  in  electrolysis,  I  shall,  in 
what  follows,  continue  to  use  the  method  of  nomenclature 
already  in  use  for  the  ions,  in  the  absence  of  a  better  system.* 

II. — METHODS  USED  IN  THIS  INVESTIGATION. 

In  looking  about  for  some  means  of  determining  the  relative 
affinities  of  the  metals  for  cyanide  solutions,  I  long  ago  came 
to  the  conclusion  that  the  determination  of  the  relative  electro- 
motive forces  of  the  metals  in  solutions  of  different  strengths 
was  the  simplest,  readiest,  and  most  certain  that  could  be 
selected.  For,  properly  considered,  it  shows  the  actual  ten- 
dency of  the  metal  to  go  into  solution.  My  first  experiments 
were  made  in  this  direction  in  August,  1896.  I  made  at  that 
time  a  large  number  of  preliminary  determinations,  the  results 
of  which  were  presented  in  a  lecture  given  February  1,  1897, 
before  the  California  Academy  of  Sciences,  in  San  Francisco. 
At  that  time,  the  curves  shown  in  Fig.  5  were  projected  on  the 
screen  by  a  stereopticon  before  an  audience  of  300  persons. 

*  H.  C.  Jones,  Z.  f.  Phys.  Ch.,  xiv.,  346,  gives  some  interesting  determinations 
of  the  EMF  of  the  combination  Ag,  AgNO3  Aq,  AgNU3,  ethyl-alcohol,  Ag, 
which  seem  to  show  that  the  solution-pressure  P  may  not  be  a  constant  for  a 
given  temperature,  but  may  also  be  a  function  of  the  solvent. 

See  also  J.  J.  Thomson,  Phil.  Mag.,  xxxvi.,  320,  on  the  action  of  the  dielectric 
in  bringing  about  dissociation  by  its  inductive  influence. 

For  other  attempts  to  explain  the  influence  of  the  solvent,  see  Bredig,  Z.  /. 
Phys.  Ch.,  iv.,  444  (1889),  "Kinetic  Nature  of  Osmotic  Pressure;"  also  Noyes, 
Id.,  v.,  53  (1890),  and  Kistiakowsky,  Id.,  vi.,  115  (1890),  " Specific  Attractions 
in  Salt  Solutions." 


28  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

The  results  of  these  experiments  have  filled  me  with  con- 
stant surprise,  when  I  have  noticed  what  apparently  slight 
causes  were  capable  of  making  great  changes  in  the  electro- 
motive force  of  the  same  metal.  The  great  delicacy  of  the 
method  proved  to  be  the  chief  source  of  difficulty  in  its  appli- 
cation, while  at  the  same  time  it  reported  faithfully  the  facts  as 
they  exist  in  nature. 

Two  methods  have  been  used  in  these  determinations,  the 
first  being  what  I  have,  for  brevity,  called  the  "  Deflection  " 
method,  and  the  other  the  "  Compensation  "  or  "  Zero  "  method 
of  Poggendorf. 

In  each  case  an  electrolytic  cell  is  constructed  with  two 
electrodes,  each  immersed  in  a  separate  solution.  One,  con- 
sisting of  the  metal  to  be  tested,  was  held  in  the  points  of  a 
platinum-tipped  pair  of  forceps,  electrically  connected  with  a 
galvanometer,  and  was  immersed  in  a  vessel  containing  the 
cyanide  solution  of  the  given  strength.  The  other  was  in  all 
cases  the  u  normal "  electrode  of  Prof.  Ostwald,  consisting  ot 
mercury,  electrically  connected  with  the  galvanometer  by 
means  of  a  glass-coated  platinum  wire.  The  surface  of  the 
mercury  is  covered  with  a  layer  of  mercurous  chloride,  a 
couple  of  inches  thick ;  and  a  solution  of  chloride  of  potassium 

M 

of  one  gramme-molecule  —  (in  this  case  also  a  normal  solution). 

The  two  vessels  containing  the  electrodes  are  connected,  as 
shown  in  Fig.  3,  by  means  of  the  tube  C  and  the  siphon  D,  the 

M 

latter  being  filled  with  —  KC1  solution,  like  that  in  the  normal 

electrode.  I  have  added  a  small  tube  E,  ordinarily  closed  with 
a  cork,  for  the  purpose  of  displacing  at  intervals  the  solution 
in  the  siphon  D  with  fresh  KC1  solution,  to  avoid  the  diffusion 
of  the  cyanide  solution  through  the  latter  back  into  the  normal 
electrode.  For  the  same  reason  the  position  of  the  normal 
electrode  is  ordinarily  a  little  higher  than  that  shown  in  the 
figure,  so  that  any  accidental  action  of  the  siphon  shall  be 
rather  away  from  the  normal  electrode  than  into  it. 

The  purpose  of  the  normal  electrode  of  Ostwald  is  to  have 
a  non-pblarizable  electrode  in  a  solution  of  known  strength  and 
electromotive  force.  This  is  fixed  at  — 0.560  volts.  That  is,  in 
the  case  of  the  normal  electrode,  the  quicksilver  ions  tend  to 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


precipitate  themselves  on  the  surface  of  the  mercury,  and  the 
solution  is  therefore  negative  to  the  metal  by  0.560  volts.  That 
is,  the  positive  current  tends  to  flow  through  the  solution  to  the 
mercury,  which  becomes  positively  electrified,  while  the  solu- 
tion itself  becomes  negatively  electrified. 

Now,  if  we  neglect  the  slight  electromotive  force  due  to  the 
contact  of  the  two  solutions,  the  resulting  electromotive  force 


Fig.  3 


SPRING  FORCEPS 

HOLDING 
METAL  STRIP  TO   BE  TESTED 


DISH    HOLDING  CYANIDE  SOLUTION, 
NORMAL   ELECTRODE  AND  METAL  STRIP. 


OSTWALD'S  NORMAL  ELECTRODE 


of  the  combined  cell  is  the  algebraic  sum  of  the  electromotive 
forces  active  at  the  two  electrodes.  Hence,  if  we  subtract 
0.560  from  the  EMF  of  the  cell,  we  have  the  EMF  of  the 
metal  under  consideration.  The  algebraic  sign  indicates  the 
direction  of  the  positive  current. 

The  Deflection  Method. — This  method  is  much  the  most  con- 
venient for  such  investigations,  particularly  in  the  first  rough- 
ing-out  of  a  large  amount  of  material.  With  proper  precau- 


30 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


tions,  it  gives  results  not  less  reliable  than  those  of  the  zero 
method ;  and  it  has  the  great  advantage  over  the  latter  that  the 
rapid  changes  of  electromotive  force  may  be  followed  almost  as 
they  occur. 

The  method  is  illustrated  in  Fig.  4.  B  is  the  cell  contain- 
ing the  cyanide  solution  and  the  metal  M  to  be  tested ;  NE 
is  Ostwald's  normal  electrode ;  R  is  a  resistance  which  varied 
in  the  tests  from  30,000  to  200,000  ohms ;  G  is  a  Wiedemann 
reflecting  galvanometer;  K,  a  make-and  break-circuit  key ;  and 
C,  a  commutator. 

Fig.  4 


THE    DEFLECTION -METHOD. 


B,       Cell  Containing  Cyanide  Solution ; 

M,       Metal  to  be  Tested; 

NE,    Gstwald's  Normal  Electrode; 


R,    Resistance  of  from  30,000  to  200,000  Ohms ; 
G,    Wiedemann's  Reflecting  Galvanometer; 
K,    Make  -  and  Break  -  Circuit  Key. 


The  galvanometer  was  calibrated  by  replacing  the  cells  B 
and  NE  with  a  Latimer-Clark  cell,  prepared  according  to  the 
directions  of  Ostwald,  and  noting  the  deflection  produced  by  its 
voltage  through  the  given  resistance  of  30,000  to  200,000 
ohms.  The  voltage  was  taken  as  EMF  =  1.438  —  0.001 
X(<°~16°  C.)  volts. 

Most  of  the  concentrations  of  potassium    cyanide  were  — 

(one  gramme-molecule,  65  grammes  per  liter,  or  6.5  per  cent.), 
or  fractional  multiples  of  this  in  tenths.     Thus  the  series  used 


was  frequently 


M  M     M       M 


M 


M 


M 


1    10   100   1000   10,000   100,000   1,000,000 
As  there  is  no  little  difference  in  the  methods  of  notation  in 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  31 

use,  and  much  resulting  confusion,  the  following  methods  of 
notation  will  always  be  used  in  this  paper :  We  shall  follow  the 
motion  of  the  positive  ions  through  the  solution,  and  the  mode 
of  notation  will  depend  entirely  on  that.  When  the  motion  of 
the -positive  ion  in  the  solution  is  from  the  metal  to  the  solution, 
the  metal  is  said  to  be  electropositive,  because  it  gives  up  posi- 
tive ions  to  the  solution  and  causes  the  solution  to  become  posi- 
tively electrified,  itself  becoming  at  the  same  time  negatively 
electrified.  Such,  for  instance,  is  the  case  of  zinc  in  a  solution 
of  zinc  sulphate.  When,  on  the  other  hand,  the  solution  gives 
up  positive  ions  to  the  metal  immersed  in  it,  as  is  the  case  with 
copper  in  a  solution  of  copper  sulphate,  the  metal  is  said  to  be 
electronegative,  for  it  causes  the  solution  in  which  it  is  im- 
mersed to  become  electronegative,  itself,  at  the  same  time,  be- 
coming positively  electrified.  The  quicksilver  in  Ostwald's 
normal  electrode  is  another  example.  The  -j-  or  —  sign,  then, 
here  indicates  the  direction  of  ionic  motion,  and  simply  shows 
whether  the  given  positive  ions  tend  to  flow  away  from  the 
metal  into  the  solution  or  towards  the  metal  from  the  solution. 
That  is,  whether  the  "  solution-pressure  "  of  the  metal  is  greater 
or  less  than  the  "  osmotic  pressure  "  of  the  ions  in  solution.* 

Now,  when  an  electropositive  and  an  electronegative  metal 
are  coupled,  the  direction  of  flow  of  the  ions  of  both  through  the 
solution  is  the  same,  and  the  electromotive  force  of  the  com- 
bination is  the  arithmetic  sum  of  those  of  the  ingredients. 
When  two  electropositive  or  two  electronegative  metals  are 
coupled,  the  ions  tend  to  flow  through  the  solution  in  opposite 
directions;  hence,  the  electromotive  force  of  the  combination 
is  equal  to  the  arithmetical  difference  between  the  separate 
electromotive  forces,  the  direction  of  motion,  and  hence  the 
sign,  being  that  of  the  greater. 

In  combinations  in  which  the  Ostwald  normal  electrode  is 
one  member,  we  know  the  amount  and  direction  of  one  elec- 
tromotive force ;  and  hence,  when  we  measure  that  of  the  com- 
bination, it  is  easy  to  calculate  that  of  the  other  (neglecting 
the  slight  electromotive  force  due  to  the  contact  of  the  solu- 
tions)^ 

*  With  regard  to  the  anions,  the  +  and  —  signs  have  an  inverse  meaning. 

f  This,  except  in  cases  of  great  differences  in  the  concentration  of  the  solu- 
tions, has  been  shown  to  cause  an  error  of  only  a  few  thousandths  or  hundredths 
of  a  volt. 


32  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

Thus,  if,  against  the  normal  electrode,  aluminum  in  a  x  . 

solution  of  KCy  gives  an  EMF  =  +  1.55  volts, — that  is,  if  the 
current  flows  from  the  aluminum  to  the  mercury,  the  same  as 
in  the  case  of  mercury, — it  follows  that  the  EMF  of  the  alumi- 

M 

num  in  -    KCy  will  be  +  1.55  —  0.560  =  +  0.99  volts. 

Again,  if  a  strip  of  amalgamated  zinc  under  similar  circum- 
stances gives  a  voltage  of  +  1.49  volts,  the  EMF  of  amalga- 

M 

mated  zinc  in  a  —  KCy  solution  will    be  +  1.49  —  0.560  = 

+  0.93  volts. 

In  making  the  determinations,  it  must  be  evident  from  the 
formula  that,  if  there  are  few  ions  of  the  given  metal  present 
in  the  solution  at  the  start,  the  introduction  of  a  very  few  more 
will  make  great  changes  in  the  value  of  the  EMF. 
p 

For  in  log.  -  it  must  be  evident  that,  as  P  is  constant  (for  a 

given  temperature),  the  value  will  depend  entirely  on  p  ;  and 
the  smaller^  is, the  greater  will  be  the  effect  due  to  slight  changes 
in  p.  Hence,  it  will  be  impossible  to  get  constant  values  for 
the  EMF,  unless  the  value  of  p  is  nearly  constant;  that  is,  when 
the  solution  is  saturated  with  ions  at  the  given  temperature. 
That  is  the  case  with  the  normal  electrode,  where  the  mercury 
lies  in  a  saturated  solution  of  mercurous  chloride.  The  mer- 
cury is  thus  in  equilibrium  with  its  ions,  and  a  constant  EMF 
results. 

To  get  perfectly  constant  results  with  cyanide  solutions,  it 
would  be  necessary  to  have  the  solution  saturated  with  the 
cyanide  of  the  metal  in  question.  But  while  this  would  give 
us  a  very  satisfactory  electromotive  series,  it  would  not  give  us 
a  measure  of  the  action  of  the  unsaturated  cyanide  solution, 
just  as  it  acts  on  the  ores.  We  must,  therefore,  be  content  with 
results  that  are  not  entirely  concordant,  and  take  the  best  of  a 
large  number  of  determinations. 

The  strips  used  were  always  freshly  burnished  with  sand- 
paper, cooled,  and  touched  to  a  grounded  platinum  wire  to  dis- 
charge any  electricity  with  which  they  might  have  been  charged 
in  burnishing. 

Preliminary  Results  with  the  Deflection  Method. — The  following 
preliminary  results  were  obtained  in  October  to  December,  1896, 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


33 


TABLE  I.— Electromotive  Force  of  Metals  in   Cyanide  Solutions. 

Deflection  Method.     Preliminary  Experiments. 

October-December,  1896. 


Ostwald's  Normal  Electrode  =  —  0.560  Volts. 
/    * 

^KCy                 £  KCy 

IBK(* 

»*<* 

Volts. 

Volts. 

Volts. 

Volts. 

•  *  Aluminum    .  ... 

+0.99 
^  -fO.93 
Not  detenu. 
+  0.81 
+0.61 
+  0.55 
'+0.45 
+0.45 
+0.39(?) 
+0.:V7 
+0.33 
+  0.29  (?) 
+0.13 
Not  deter  m. 
Not  determ. 
—0.09 

+0.90 
+0.82 
+  0.77 
+  0.62 
+0.57 
-i-0.31 
-f  0.24 
+  0.25 
-t-0.41 
+0.23 
+0.15 
+0.25 
+  0.05 
+0.01 

+0.76 
+0,70 
+0.59 
+  0.37 
+0.35 
+0.19 
+0.17 
—0.16 
-0.14(?) 
+0.09 
-  O.Oo 
+0.05 
+  0.01 
—0.07 
—0.03 
—0.1  1 
—0.13 
—0.03 
—0.21 
—0.20 
—0.44 
—0.24 
—0.44 
—0.48 
-0.52 
—  0.55(?) 
—0.52 
—0.50 
—0.54 
—0.52 
—0.54 
—0.50 
—0.57 
—0.57 
—0.55 
—0.55 
—0.56 
—  0.42(?> 
—0.54 
—0.52  (?) 
—0.56 

+  0.40 
+  0.44 
+  0.39 
+0.16 

*Zinc,  amalgamated 

*Zinc,  commercial 

*Copper  

*Cadmium  

Cadmium,  amalgamated 
*Tin  

+0.06 

*Bornite  

Copper,  amalgamated 
*Gold  

—  0.12(?) 
—0.38 
-0.36 
-0.44 

'Silver  

1  *Copper-Glance  

i  *Lead  

Tin,  amalgamated 

—0.12 

Lead,  amalgamated  

^Quicksilver  

+0.01 

Gold,  amalgamated 

—0.^6 

*Antimonv  

+0.06 
+0.04 
+  0.00 
—0.1  1 
-0.17 
—0.20 
—  0/28 
-0.28 
—0.28 

—  O.XO 

030 

. 
+0.03 
—0.05 
—0.06 
—0.17 
—0.24 
—0.34 
—0.42 
—0.48 
056 

1  *Arsenic  

*Bismuth  

Niccolite  

*Iron  

*Chalcop  vrite  



*Pvrite..."  



*Galena  



*Argentite  

Berthierite  

—0.52 
—0.33 
—0.40 
—0.52 
—0.45 
—0.46 
—0.55 
—0.52  (?) 
—  C..V2 
—0.55 
—0.55 
-0.52 
—0.53  (?) 
—0.55 
—0.56 

Speisscobalt  

|  Magnetopyrile  

—0.30 
—0.36 
—0.40 
—0.40 
—0.43 
-0.46 
—0.48 
—0.50 
—0.50 
-0.52 
—0.54 
—0.54 
—0.56 

Fahlore  

!  Arsenopyrite  

^Platinum  

Cuprite  

*Electric  Light  Carbon.. 
*Blende  

Boulangerite  

Bournonite  

Coke  

Ruby  Silver-Ore  

Stephanite  

*Sdbnite    . 

with  some  of  the  common  metals  and  minerals.  The  metals 
were  good  commercial  articles,  such  as  are  in  use  in  the  arts, 
except  in  the  case  of  gold,  silver  and  quicksilver,  which  were 
chemically  pure.  In  the  case  of  some  of  the  minerals,  such  as 
zincblende,  stibnite,  etc.,  the  electrical  resistance  was  probably 

3 


34 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


so  high  in  comparison  with  that  of  the  intercalated  resistance 
that  the  results  may  be  somewhat  low. 

Nevertheless,  they  give  at  once   some  important   relations 

Fig.  5 


PRELIMINARY  RESULTS.     OCT.  -  DEC.  1896. 

which  must  exist  whenever  the  cyanide  process  is  applied  in 
the  treatment  of  ores. 

The  electromotive  forces  of  the  metals  and  minerals  marked 
with  an  asterisk  in  the  above  table  have  been  plotted  in  Fig.  5. 
The  Y  axis  shows  the  potential  in  volts,  the  X  axis  the  con- 
centration in  gramme-molecules  and  also  in  percentage  of  KCy. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  35 

It  will  be  noticed  that  in  most  cases  the  curves  approximate 
quite  closely  to  the  logarithmic  curve  which  theory  would  give 
(see  Fig.  2),  supposing  the  osmotic  pressure  of  the  metallic  ions 
present  to  be  inversely  proportional  to  the  concentration  of  the 
free  potassium  cyanide  present;  but  they  have  different  origins. 

It  will  be  noticed  that  the  electromotive  force  of  commercial 
sheet-zinc  is  increased  by  amalgamation,  probably  by  reducing 
local  action  with  some  of  its  impurities,  by  which  some  of  the 
current  produced  is  short-circuited.  In  all  the  other  experi- 
ments, amalgamation  reduces  the  electromotive  force  of  the 
combination. 

With  some  substances,  particularly  aluminum,  copper,  iron, 
platinum  and  gas-carbon,  it  was  very  difficult  to  get  concordant 
results ;  with  aluminum  and  copper  this  seemed  to  be  due  to 
a  tendency  to  form  an  insoluble  film  on  the  surface  of  the 
metal,  which  put  a  stop  to  further  action.  With  copper  and 
iron  it  was  also  possibly  due  to  a  tendency  of  the  metals  to  a 
change  of  valency,  which  is  accompanied  by  a  change  in  the 
electrical  state.  With  platinum  and  gas-carbon,  it  was  not  im- 
probably due  to  a  varying  content  of  absorbed  gas. 

In  testing  the  minerals,  it  was  in  all  cases  difficult  to  get  a 
complete  electrical  contact  between  the  tips  of  the  platinum 
forceps  and  the  rough  surface  of  the  mineral  fragment,  so  that 
the  results  are  only  provisional,  particularly  as  the  resistance  in 
some  of  these  cases  was  very  high.  Nevertheless,  the  results 
are  very  interesting.  They  show,  for  instance,  that  not  all 
copper  minerals  have  a  strong  action  on  the  current.  Pure 
chalcopyrite,  for  instance,  has  hardly  more  action  than  pure 
pyrite,  while  bornite  and  copper-glance  have  a  very  decided 
tendency  to  go  into  solution.  Cuprite  is  also  apparently  very 
little  acted  on,  though  this  may  be  due  to  its  high  resistance 
rather  than  to  a  lack  of  tendency  to  dissolve.  The  soluble  salts 
and  minerals  of  copper  could  not  be  tested  in  this  manner,  ow- 
ing to  their  non-conductivity. 

It  is  plain,  however,  that  pure  chalcopyrite,  galena,  argen- 
tite,  magnetopyrite,  fahlore,  arsenopyrite,  blende,  boulangerite, 
bournonite,  ruby  silver-ore,  stephanite  and  stibnite,  when  free 
from  their  oxidation-products,  are  apparently  very  little  acted 
on  by  cyanide  solutions. 

It  is  also  plain  that  a  particle  of  metallic  gold,  in  contact  with 


36  THE    ELECTROMOTIVE    FORCE    OF    METALS. 


a  particle  of  pyrite,  forms  a  galvanic  couple  in  -  -  or  6.5  per 

M 

cent.  KCy  solution,  equal  to+  0.65  volts  :  in  —  or  0.65  per  cent. 

KCy  solution,  +  0.65volts,  and  in  —  or  0.065  per  cent,  KCy  so 

lution,  +  0.57  volts.    With  zinc  under  the  same  circumstances  (if 

M 

we  take  for  the  --  KCy  solution  the  figures  for  amalgamated 

zinc),  taking  the  zinc  as  the  more  electropositive  metal,  and 
subtracting  the  potential  of  gold,  we  have  differences  of  +  0.56 
volts,  +  0.54  volts,  and  +  0.50  volts.  In  short,  these  figures 
would  measure  the  tendency  of  the  zinc  to  dissolve,  or  of  the 
gold  to  precipitate  in  KCy  solutions  of  these  strengths. 

According  to  these  figures,  the  precipitating  power  of  the 
zinc  seems  to  hold  up  quite  well  for  the  dilute  solutions.  The 
actual  failure  to  precipitate  the  gold,  sometimes  met  with  in 
dilute  solutions,  is  no  doubt  due  to  films  of  cyanide  or  hydrate 
of  zinc,  which  form  incrustations  on  the  surface  of  the  zinc  and 
thus  prevent  contact.  The  fact  that  the  use  of  a  small  amount 
of  fresh  cyanide  or  of  caustic  potash  in  the  zinc-boxes  starts 
precipitation  again,  seems  to  favor  this  explanation. 

The  Zero-Method.  —  This  method  is  shown  in  outline  in  Fig. 
No.  6.  KE  is  the  Ostwald  normal  electrode.  B  is  the  cell 
containing  the  cyanide  solution  in  which,  as  before,  is  immersed 
the  metal  M  to  be  tested.  At  G  is  a  galvanometer.  At  R  is  a 
resistance,  graduated,  in  my  experiments,  into  10,000  parts.  A 
storage-battery  of  two  volts  and  the  combination-cell  1N"E-B  are 
so  connected  that  their  positive  poles  are  both  connected  at  the 
same  end  of  the  resistance  R.  The  negative  pole  of  the  storage- 
battery  is  attached  to  the  other  end  of  the  resistance  R,  so 
that  the  whole  current  of  the  storage-battery  discharges  con- 
stantly through  R.  The  latter  should  be  great  enough  to 
avoid  heating,  and  to  maintain  a  constant  potential  between  the 
ends  of  R,  The  other  terminal  of  the  combination  (the  nega- 
tive pole)  is  then  moved  along  the  resistance  R  till  some  dis- 
tance, a,  is  reached  at  which  the  EMF  force  of  KE-B  is  exactly 
balanced  by  the  EMF  force  of  the  storage-battery  for  that  frac- 
tion of  R  represented  by  a.  In  this  case  there  is  no  deflection 
of  the  galvanometer  ;  at  other  points  the  galvanometer  will  be 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  37 

deflected  either  to  the  right  or  left,  according  as  too  much  or 
too  little  EMF  is  used  to  balance  KE-B.  The  EMF  of  the 
storage-battery,  is,  of  course,  first  calibrated  by  comparing  it 
with  a  standard  Latimer-Clark  cell,  placed  where  NE-B  is. 

This  method  of  determining  the  EMF  of  a  cell  is  deservedly 
considered  one  of  the  most  reliable.  With  non-polarizing  cells, 
it  certainly  leaves  nothing  to  be  desired.  But,  in  investigations 
of  this  kind  with  cells  that  are  easily  polarized,  accurate  results 
are  obtained  only  by  a  long  number  of  very  tedious  approxima- 
tions, which  render  the  work  almost  interminable.  For  it  is, 
of  course,  impossible  to  hit  the  right  balance  at  first ;  and,  if 
the  connection  is  made  at  any  point  except  the  right  one,  the 

Fig.  6 


STORAGE   CELL 
2    VOLTS 


POGGENDORPS    COMPENSATION  -  METHOD. 

B,       Cell  Containing  Cyanide  Solution;        R,    Resistance  Graduated  into  10,000  Parts; 

M,       Metal  to  be  Tested;  V,    Movable  Contact; 

KE,    Ostwald's  Normal  Electrode;  G,    Wiedemann's  Reflecting  Galvanometer. 

metallic  electrode  will  receive  either  a  positive  or  negative  charge 
from  the  storage-battery,  and  a  true  reading  will  be  thus  made 
impossible.  It  is  necessary  to  change  the  entire  solution  in  B, 
put  in  new  electrodes  at  M,  drive  out  the  diffused  cyanide 
solution  from  NE,  and  so  on,  till  these  operations  have  been 
repeated  perhaps  a  dozen  times.  If  this  is  not  done,  the  results 
are  very  unreliable.  With  the  deflection  method,  on  the  other 
hand,  the  observations  may  be  made  very  rapidly,  and  though 
there  is  a  tendency  for  the  readings  to  be  a  little  low  unless  they 
are  quickly  made,  still,  with  a  high  intercalated  resistance,  and 
a  delicate  reflecting  galvanometer,  this  method  seems  to  be  re- 
liable for  these  quickly  polarizing  electrodes. 

As  I  have  already  stated,  and  as  was  first  pointed  out  by  Ost- 


38 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


wald,  strictly  concordant  results  are  possible  only  when  the 
electrode  is  surrounded  with  a  medium  already  saturated  with 
its  ions. 

I  had  intended  to  verify  the  results  in  Table  I.  with  the  zero 
method  before  publication,  but  although  I  had  all  the  apparatus 
set  up  for  over  two  years,  ready  to  begin  at  any  time,  I  was 
prevented  by  the  constant  pressure  of  routine-work  from 
touching  it,  till  shortly  before  the  time  set  for  the  San  Francisco 
meeting  of  the  Institute,  in  September,  1899.  Meantime  Prof. 
A.  von  Oettingen,  professor  of  physics  in  the  University  of 
Leipzig,  read  a  very  valuable  paper  on  this  subject  before  the 
Chemical  and  Metallurgical  Society  of  South  Africa,  in  Jan- 
uary and  February,  1899.  In  this  paper  he  gives  the  results  of 
a  large  number  of  determinations  which  he  made  of  the  elec- 
tromotive force  of  metals  in  cyanide  solutions  by  means  of  Pog- 
gendorf  s  compensation  method,  or,  as  I  shall  call  it  for  brev- 
ity, the  zero  method. 

Professor  von  Oettingen's  results  are  given  in  Table  II. 

TABLE  II. — Potentials  of  Efferent  Metals  in   Contact  with  KCy 
Solutions,  at  25°   C. 

Experiments  of  Prof.  A.  von  Oettingen,  Jour.  Chem.  and  Metallurgical  Soc.  S.  Africa, 
January  and  February,  1 899. 


»KCy 

M 
-   KCy 

10 

100 

-M-  KCy 

1000 

S)KCy+767oAu 

Volts. 

Volts. 

Volts. 

Volts. 

Volts. 

Au  

J  +0.340  to 
(  +0  306 

f  +0.180  to 

1  +0.218 

(  —0.092  to 
\  —0.056 

(-0.414  to 
1-0474 

—0.020  to  —0.170 

As 

f  +n.330  to 

+0.176  const. 

—0.020  const.   /—  0.340  to 

—0.308  to  —0.330 

•™B  

t  +0.314 

1  —0.200 

Cu 

j  +0.890  to 

J  —0.680  to 

f  —  0.212*  to 

f  —0.550  to 

i  0924 

1  +0.648 

t  +0.380                1  —0.230 

He- 

+  0.16v  to 

(  +0.008  to 

+0.056  const. 



"6  

+0.200 

1  +0.024 

\i 

1    

—0.290  to 

j—  0.466  to 

f  —0.550  to 

-0.560 

+0194 

1  —0.392 

1  —0.488 

Co 

j  +0.182  to 

(+0.118  to 

f—  0.1  68  to 

1    j-0196 

1  —  0.220f 

1  -0.240 

p, 

f  +0.056  to 

(  +0.034  to 

(  —  0.0">4  to 

j  —0.008  to 

1  —0.146 

1—0.012 

1  +0.022 

1  +0.050 

FeoO  , 

j—  0.674  to 

(—0.796  to 

j—  0.824  to                     

r  "2^:1  •  ••• 

1  —0  700 

•j  —0.720 

1  —0.750 

PbO,  

+0.160  const. 

j  +0.110  to 

(-0.062  to 

—0.006  const. 

1+0.118 

1  4  0.070 

Pb  

+0.164  const. 

+0.128  const. 

+0.120  const. 

+  0.120  const. 

+0.126  const. 

Zn  

(  +0.924  const. 
(  +0.940 

f  +0.780  to 
t  +0.800 

(  +0.560  to 
t  +0.604 

+0.480-  const. 

*  On  moving  the  fluid  the  potential  suddenly  rises. 

t  The  potential  changed  suddenly  from  — 0.121  to  +0.118,  then  remained 
constant. 

(Note  the  discrepancy  between  —0.220  in  the  table  and  —0.121  in  the  foot- 
note. ) 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  39 

The  above  results  were  all  obtained  by  the  Poggendorff 
compensation  or  zero  method,  the  Lippman  capillary  electro- 
meter being  used  as  an  indicator  instead  of  a  galvanometer. 

Prof,  von  Oettingen  says  of  these  results  : 

11  The  two  figures  in  each  column  refer  to  the  first  and  last  observations  on  each 
metal,  the  intermediate  values  being  omitted.  The  time  occupied  by  the  change 
is  very  variable  ;  Cu,  for  instance,  took  an  hour.  When  no  changes  occur,  this  is 
indicated  by  a  constant.  The  changes  of  potential  are  not  always  in  the  same  di- 
rection ;  sometimes  decreasing,  sometimes  increasing.  But  the  direction  of  the 
changes  in  any  given  metal  is  always  the  same." 

It  will  be  noticed  on  examining  the  table  that  this  last  sen- 
tence is  not  correct  (unless  there  should  be  a  typographical 
error  in  his  table).  For  gold,  silver,  copper,  cobalt,  ferric  oxide 
and  lead  peroxide,  the  highest  value  for  the  same  metal  is  some- 
times the  first  and  sometimes  the  second  value.  In  the  case 

of  copper  in  -^-—  KCy  the  results  jump  from  —  0.212  volts  to 

-f  0.380  volts—  a  difference  of  0.592  volts.     I  shall  speak  of 
the  probable  cause  of  these  differences  later.* 

*  There  are  some  other  potential  differences  given  by  Prof,  von  Oettingen  which 
I  include  here. 

-  =  —0.99  volts.     (Ostwald),  ^  Solutions. 


—  -4-0  VM         "  "  " 

ZnS04  ~ 

Mg     =  +1.243      "  "  "  18°  C. 


MgS04 


Pb  =-0.089 


Pb  (C,HA)i 

Cu 


=  —0.582  volts. 


CuSO4 

Arr 

-I 


-      g-  =  — 0.5CO  (Ostwald's  normal  electrode.) 
Hg,Cl2 
Prof,  von  Oettingen  himself  determined  the  following  also  (all  at  25°  C.). 

__^_  -  =  —1.64  to  1.42  volts,  variable. 
AuCl  (cone.) 

Au 
•yAnCl3  =  — 1.134  volts. 

— ~"-r    =—1.09  VOltS. 

Aqua  Kegia 


40  THE    ELECTROMOTIVE   FORCE    OF    METALS. 

In  order  to  make  more  clear  the  meaning  of  Prof,  von  Oet- 
tingen's  results,  I  have  plotted  them  in  Fig.  7  as  mine  are 
plotted  in  Fig.  5.  In  the  figures,  x  is  made  to  mark  the  molec- 

ular concentration  ^_,  _    —  -,  ;  the  y  axis  shows  the  po- 


tential in  volts.  The  designation  Zinc  1  means  that  this  was 
the  first  value  obtained  with  zinc,  the  designation  Zinc  2,  the 
final  value,  etc.  It  will  be  noticed  that  sometimes  the  first 
value  is  higher  than  the  second  and  sometimes  vice  versa  ;  but 
the  results  are  not  consistent  throughout,  sometimes  crossing 
each  other. 

The  first  curves  of  each  metal,  except  mercury,  approximately 
follow  the  logarithmic  law  (on  the  assumption  that  the  num- 
ber of  metal  ions  is  inversely  proportional  to  the  potassium 
cyanide  concentration).  Evidently  the  curves  will  cross  the  X 
axis  at  different  points,  and  not  usually  at  a  molecular  con- 
centration M  =  1,  unless  it  should  accidentally  happen  that 
p 
—  =  1  for  M  =  1.  The  second  curves  of  zinc,  copper,  gold 

and  silver,  also  approximately  follow  it.  But  the  second  curves 
of  mercury,  cobalt,  nickel  and  iron  depart  considerably  from 
it.  It  is  possible  that  these  departures  are  due  to  polar- 
ization effects,  as  already  explained.  The  irregularities  are 
much  more  marked  than  with  the  defiection-method.  With 
that  method,  provided  a  sufficiently  large  resistance  is  used,  the 
first  deflection  is  the  greatest,  and  is  taken  as  the  reading 
nearest  to  the  truth.  The  deflection  then  gradually  falls  (often 
quite  rapidly,  if  there  is  a  formation  of  gas  on  the  face  of  the 
electrode)  ;  but  the  electromotive  force  never  rises  unless  the 
first  effect  of  the  current  is  to  produce  a  film  of  gas  or  insoluble 


=  ~°'26  to  +0'03'  very  variable-        ~  ==  4  °'21  volts' 


Au  -J^o  =  0  to  0.136  volts. 


~  KCyS  =  -0.36,  variable  volts. 

M~^~  KCAgsa(.-r  =  -f  0.536  volts. 
—  KCyS  =  — 0.40  volts. 

A"        =+0.50  volts.  ^^ -  =  +1.154  volts. 

KCy(sat)  KCy  (sat.) 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


41 


cyanide  which  puts  a  stop  to  the  current,  either  by  setting  up 
an  opposing  EMF  or  by  preventing  or  reducing  contact  by  its 
resistance.  In  this  case,  shaking  the  solution  or  jarring  the 


41.0 

+0.9 
40.8 
+0.7 
+0.6 
+0.5 
-fO.4 
40.3 
+0.2 
+0.1 

0 
—0.1 
•  0  2 

0    10    ^                              Fig.  7.                        3 

-  KCy 

7i 

I|H 

0. 

1 

^ 

••  — 
—  -  — 

5 

^ 

.  

•7^- 

-rrr 

.  

^r- 

r 

;-'- 

/ii 
(.'<• 

lo.  1 
rNTo. 

- 

.' 

^ 

^-* 
^^  — 

^-~> 

»-.—  - 

—  - 

,  

r^ 

--  — 

'!" 

j 

Ob 

1  , 

/^ 

^ 

f* 

,' 

/ 

/ 

^' 

\l 

3£ 

\1 

// 

wf 

/ 

/.in 

•  N 

>.  1 

** 

l/j 

Goid  No. 

i 

I 

.• 

Silver,  No.  1 

> 

g 

==s 

=== 

~- 

~ 

=E 

E? 

i^ 

F^ 

rzrr: 



= 



^ 

-_ 

~- 

Sil 
OR 
H< 
O 

vei^Nc 

fcury- 
SaJt  N 
fltel~N" 

^fN 
ad  | 

., 

z^- 

h 

1 

/X 

.. 

: 



77 

i 

- 

J02W< 

^ 

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.  -  -• 

i 

= 

=^ 

^= 

^ 
.- 

^ 

«=r£ 

-^^ 

v^ 

ei 

&---* 

| 

-  .. 

<s 

Oo 

I.« 

T 

Merc^ir>j  N<j 

•  iT~^Ma^ 

_- 

--- 

"" 

—  ' 

_  —  • 

^ 

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" 

s' 

jli 
IK 

rcury 

cffl 

RS 

jil^l:^  — 

— 

--- 

- 

^n^fo 

i 

Iijn  No. 

\- 

/wr°~ 



^^- 

' 

xx 

Pb02  No  l' 

•^ 

?-*- 

v»* 

1  — 

~r. 

-^- 

-— 

——  *, 

-^, 

--, 

—  ^ 

— 

•  — 

Ir. 

ol 

IX 

|~ 

Cooalj  xA.  1 

SilVer  Noj  2- 

~*^  m 

^> 

-"• 

,' 

-0.3 

Coppet  Xo. 

' 

x 

.' 

Cobalj  N|  2 
SilyerlNo'  1- 

r 

,'' 

7^ 

^—c 

^-  — 

</- 

^-~~ 

^^ 

—  =- 

.  —  • 

— 

,  —  ' 

~^-~ 

--- 

-e=r- 

—  1  —  ' 

.  —  • 

—  .  — 

_^- 

-«  —  • 

-~r- 

Ni 

ke 

N 

— 

.  i 

1 

-0.5 
—0.6 
-0.7 

Gold  No. 

\~ 

/ 

Gold  No. 

t~ 

—  \' 

X" 

- 

i 

oppei 

N< 

.  i 

3 

'/ 

\ 

ickjel  ]j 

0. 

y 

3" 

• 

Pe 

Us 

de 

Nc 

i 

Tv" 

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At 

Re 

i 

d 

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5 

— 

.  —  — 


-   -» 

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^  —  - 

.-•  -  : 
.  • 

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_  — 
—  — 

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-=* 

^rr 



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;-v 

'X 

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1 

^  

ft 

r» 

w 

X 

.  i 

E.M.F.  OF  METALS  IN  CYANIDE -SOLUTIONS. 

by  Prof.  v.  Oettingen  of  Leipzig. 
(J.  Chem.  &  Met.  Soc.  S.  A.  Feb.  1899.) 

x-M.       y-7T-.058  log  ^    Volts. 

electrode  usually  gives  an  increase  of  the  EMF  by  destroying 
the  film  in  part;  but,  if  the  metallic  surface  is  untarnished  to 
begin  with,  the  EMF  rarely  rises  again  to  its  first  value. 

New   Method  of  Plotting  Results. — The  method  of  plotting 


42  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

results  hitherto  used,  while  it  shows  very  well  the  near  ap- 
proach of  the  curve  to  the  true  logarithmic  curve,  has  the  dis- 
advantage that  only  three  or  four  values  for  the  tenth  ratio  can 

p 
be  plotted.     If,  however,  instead  of  making  x  =  —,  as  we  have 

P  P 

done,  we  let  x  —  log.  — ,  and  plot  the  curve  y  =  0.058  log.  -  volts, 

the  curve  becomes  a  straight  line  passing  through  the  origin 

at  0.     For  x  =  0,  y  =  0. 

p 
This   curve   is   plotted  in  Fig.   8    for  values  of  x  =  log.  - 

from  4-  13   to  —  12,  which  gives   voltages   from  -j-  0.755  to 

p 
—  0.696,  arid  the  table  shows  values  from  x  =log.  —  =  minus 

P 

infinity  to  40.  It  shows  what  an  enormous  change  in  the  value 
p  m 

-  is  necessary  to  produce  a  very  moderate  change  in  the  volt- 
age.    Thus,  to  produce  a  change  of  2.32  volts,  a  change  in  the 

p 
ratio—  — 1040  (or  ten  to  the  fortieth  power)  is  necessary. 

In  our  experiments,  of  course,  we  do  not  know  the  value  of 
p 

— ,  but  as  a  first  approximation  we  may  assume  it  inversely  pro- 
portional to  the  molecular  concentration  M  X  10". 

On  the  axis  of  x  is  plotted  the  logarithm  of  the  molecular 
concentration  expressed  in  the  powers  of  10.  Thus :  x  =  log. 
M=log.  10±n.  The  y  axis  gives  the  EMF  in  volts.  For 
comparison  the  theoretic  formula  of  Nernst  is  also  given. 

If  we  plot  Prof,  von  Oettingeii's  results,  as  in  Fig.  9,  on  this 
plan,  they  become  at  once  more  intelligible.  We  see  at  once 
that  all  the  curves  do  not  remain  straight  lines.  The  zinc  fol- 
lows along  very  nearly  in  the  theoretic  straight  line.  The 
copper  starts  well,  but  soon  falls  quite  rapidly,  due  probably  to 
increasing  dissociations.  The  gold  and  silver  approximate 
fairly  well,  also;  but  the  rest  depart  from  it  considerably. 

Comparison  of  the  Deflection  and  Zero  Methods. — Since  the 
results  of  Prof,  von  Oettingen  were  published,  I  have  thought 
best  to  try  the  zero  method  as  well  as  the  deflection  method, 
and  to  compare  the  results  so  far  obtained  with  each  other.  I 
have  also  decided  to  plot  the  results  by  the  same  method  as 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


43 


shown  in  Fig.  8,  as  it  enables  us  to  compare  the  results  over  a 
wider  range  of  dilution  than  the  former  method  of  tabulation 
would  cover. 


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After  the  foregoing  description  and  discussion  of  the  various 
methods  employed  in  this  investigation,  the  reader  will  be  able 


44 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


Fig.  9. 


VOLTS 
+  1.0 


+  0.9 


—1.0 


E.M.F.  OF  METALS  IN  CYANIDE- SOLUTIONS. 

Prof.  v.  Oettingen  of  Leipzig.    (J.  Chem.  and  Met.  Soc.  S.  A.  Feb.  1899.) 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

to  study  intelligently  the  tabulated  results  of  the  tests  herein- 
after stated. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


45 


in. — RESULTS  OF  EXPERIMENTS. 

The  following  tables  show,  for  the  several  metals  tested,  my 
own  results,  obtained  at  different  times  and  by  different  meth- 
ods, as  well  as  those  of  Prof,  von  Oettingen.  In  every  case, 
each  observation  was  made  independently,  without  regard  to 
the  ultimate  result  of  its  reduction.  But  the  later  readings  are 
more  reliable  than  the  earlier,  because  a  certain  knack  in  catch- 
ing the  needle  at  its  maximum  position,  before  the  voltage  be- 
gins to  fall,  was  acquired  during  the  work.  The  tables  give 
the  readings  as  reduced  from  the  actual  observations,  without 
attempted  correction ;  but  when  any  anomaly  rendered  the  ob- 
servation uncertain,  this  is  indicated  by  a  (?).  Such  was  the 
case  particularly  in  the  readings  with  distilled  water  (M  divided 
by  oo  ),  which  were  very  uncertain,  especially  for  easily  oxidiz- 
able  metals  like  zinc  and  iron. 

TABLE  III. — Electromotive  Force  of  Zinc  (Commercial  Sheet, 
Burnished}  in  KCy. 


Curve. 

(a). 

tt». 

(c). 

(d). 

(«). 

(/)• 

(9). 

Note-book  Bl.  page- 
Date  

67 
Oct.  19,'% 
Deflect.* 
100,000 
22-J  C. 
Christy. 

Volts. 

176 
A'g.  30,'99 
Zero.f 

19*  C" 
Christy. 

Volts. 

+0.946 
+0.861 
+0.772 
+0.415 
+0.385  (?) 
+0.355 
+0.383  (?) 
+  0.372  (?) 

177 
A'g.30,'99 
Deflect.f 
100,000 
19°  C 
Christy. 

Volts 

+0.386 
+0.326 
+0.320 
+0.312 

+0.256  (?) 

186 
Sep.  4,  '99 
Deflect.! 
100,000 
19°  C. 
Christy. 

Volts. 

+0.906 
+0.815 
+0.730 
+0.300 
+0.270 
+0.270  (?) 
+0.282  (?) 
+0.240  (?) 
+0.350  (?) 

193 
Sep:  8,  '99 
Deflect. 
200,000 
198  C. 
Christy. 

Volts. 

+0.914g 
+0.836§ 
+0.735? 
+0.371  B 
+  0332 
+0.332  (?) 
+0.332  (?) 
+0.293  (?) 

Publsh'd. 
Feb.,  '99 
Zero. 

Publsh'd. 
Feb.,  '99 
Zero. 

Method 

Resis.  ohms  
Temperature  
Observer  

25°  C. 
Von  Oet- 
tingen. 
Volts. 

+0.924 
+  0.780 
+0.560 
+0.480 

25°  C. 
Von  Oet- 
tingen. 
Volts. 

+0.940 
+0.800 
+0.604 
+0.480 

EMF 

(N.  E.  =  —  0.560)  
Concentration  : 

KCy  ^ 

*  r 

M 

-1-0.770 
+0.585 
+0.385 

10 

M 

-      100 
M 

1,000 
M 

10,OUO 
M 

100,000 
M 

1,000,000 

M  /       TT  r\\ 

-1-0.041  (?) 

»(       H20)  

*  Used  same  strip  of  zinc  throughout  experiments,  burnishing  each  time. 
Tested  from  strong  to  weak  solutions.  t  Used  new  burnished  strip  each  time. 

t  Same  strip,  burnished  each  time.     Tested  from  weak  to  strong  solutions. 

2  Fine  bubbles  form  on  zinc  and  then  voltage  falls.  Shaking  causes  bubbles 
to  escape,  and  voltage  rises.  ||  Below  this,  no  gas-bubbles  visible  to  naked  eye  ; 
but  voltage  falls,  and  then  rises  on  shaking. 


46 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


Experiments  with  Commercial  Sheet-Zinc. — Experiments  were 
made  at  different  times,  with  both  the  zero  and  the  deflection 
method,  on  the  ordinary  commercial  sheet-zinc,  such  as  is 
actually  used  in  making  zinc-shavings  for  precipitating  gold. 
The  results  are  given  in  Table  III.,  and  those  of  Prof,  von 
Oettingen  have  been  introduced  into  the  same  table,  for  com- 
parison. 

These  results  are  plotted  together  for  comparison  in  Fig.  10. 

It  will  be  evident  that  from ---to  --- ,  or  from  6.5  to  0.065  per 

Fig.  10. 


1      1 

VOLTS 
4-1.0 

40.9 
40.8 
40.7 
40.6 
40.5 
40.4 
40.3 
40.2 
40.1 
0 

3.        Defl.  22°C.  Christy 
b         Zero.  19  JC.  Christy 
C.        Defl.  19  'C.  Christy 
d.        Defl.  19°C.  Christy 
e          Defl.  19"  C.  Christy 
i^g      Zero.  25  C.  v.  Oettingen 
X  =     log  M 
y  =77"=  0.058  log  ^p  Volts 

g 

^- 

^•— 

t 

>""^ 

^^ 

^ 

^ 

^ 

'3 

-  L 

? 

^ 

'/, 

-^ 

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— 

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O-I 

d^ 

=,-  ^ 

1 

3      OD 

I—G 

-5 

-4 

^ 

-1 

E.M.F.  OF  ZINC  IN  KCy.  SOLS. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M=  log.  10n ;   on  the 
vertical  (y)  axis,  the  actual  volts. 

cent.,  the    curve    nearly    follows    the    theoretic    straight   line. 

M 

Curves  a,  /  and  g  appear  to  follow  it  to  or  0.0065  per 

cent,  but  for  more  dilute  solutions  beyond  that  point  the  curve 
approximates  a  horizontal  straigh  tline.  This,  according  to  the 
Nernst  theory,  would  mean  that  the  number  of  zinc  ions  in  such 
solutions  remains  nearly  constant.  In  spite  of  all  the  irregu- 
larities in  the  curves,  the  point  —  3  or  _  -or  0.0065  per  cent. 
KCy  is  evidently  a  critical  or  inflection-point  in  the  curve. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


47 


The  results  obtained  with  high  dilutions  of  cyanide  and  with 
distilled  water  were  very  uncertain,  probably  because  of  the 
formation  of  insoluble  films  of  oxide  of  zinc  and  occluded  hy- 
drogen, which  prevented  the  accurate  reading  of  the  needle. 

TABLE  IV. — Electromotive  Force  of  Copper  (Burnished  Sheet) 

in  KCy. 


Curve. 

(a). 

(&). 

(c). 

<«. 

M. 

(/). 

(9). 

Note-book  Bl.  page- 
Date      

68 
Oct.  20/96 
Deflect. 
100,000 
23°  C. 
Christy. 

Volts. 

+0.930 
+0.620 
+0.370 
+0.158 

178 
A'g.30,'99 
Zero. 

is°6.' 

Christy. 
Volts. 

+0.910 
+0.731 
+0.146 
—0.104 
—0.332 
—0.360 
-0.426 
—0.444 

188 
Sep.  6,  '99 
Deflect. 
100.000 
18°  C, 
Christy. 

Volts. 

+0.905 
+0.310 
+0.310 
-0.068 
—0.241 
—0.299 
—0.314 
—0.328 

189 
Sep  6,  '99 
Zero. 

iF'c". 

Christy. 
Volts. 

+0.811  (?) 
+0.663 
+0.356 
-0.048 
—0.230 
—0.272 
—0.282 
—0.313 

190 
Sep  7.  '99 
Deflect. 
200,000 
19°  C. 
Christy. 

Volts. 

+0.860 
+0.660 
+0.149 
—0.151 
—0.324 
-0.387 
—0.442 
—0.450 

Publsh'd. 
Jan.,  '99 
Zero. 

25°"c" 
Von  Oet- 
tingen. 
Volts. 

+0.924 
+0.680 
-0.212 
-0.550 

Publsh'd. 
Jan..  '99 
Zero. 

25""c". 
Von  Oet- 
tingen. 
Volts. 

+0.890 
+0.648 
+0.380 
—0.230 

Method 

Resis.  ohms  

Temperature 

Observer  

FMF 

(N.  E.  =  —  0.560)  
Concentration  : 

KCy  M.... 

•^y  j 

M 

10 
M 

100 
M    .. 

1,000 
M 

10,000 
M 





100,000 
M 

1,000,000 
™(=  H20)  

—0.560  (?) 

In  my  results  with  the  deflection  method,  I  have  always  taken 
the  highest  reliable  reading  as  the  most  probable  result.  It  was 
often  quite  difficult  to  make  sure  of  the  proper  reading,  as  a 
slight  insoluble  film  of  cyanide  of  copper  formed  almost  in- 
stantly, and  this  lowered  the  potential  almost  before  a  reading 
could  be  taken.  On  agitating  the  copper,  so  as  to  bring  it  into 
contact  with  fresh  solution,  the  potential  would  gradually  rise 
to  a  maximum,  after  which,  on  being  left  at  rest,  it  would  again 
fall  off  more  gradually.  It  is  possible,  also,  that  the  tendency 
of  copper  to  form  cupric,  as  well  as  cuprous  cyanide,  may  in 
part  explain  the  discordant  results,  such,  for  instance,  as  that 

obtained  by  Prof,  von  Oettingen  with 


^ 


solution.     He 


says  in  a  footnote  concerning  this  case,  "  On  shaking,  the  poten- 
tial suddenly  rises  from  —  0.212  to  +  0.380." 


48 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


The  results  contained  in  the  above  table  have  all  been  plotted 
in  Fig.  11.     The  mean  results  of  these  curves  show  a  tendency 

to  follow  the  course  of  a  straight  line  from   -   down  to 


or  perhaps  to 


M 


10,000 


;  that  is,  from  6.5  down  to  0.00065  per 

Fig.  11 


+  1.0 
+  0.9 
-1-  0.8 
+  0.7 
+  0.6 
-1-  0.5 
+•  0.4 
+  0.3 
4-  0.2 
+  0.1 
0 
—  0.1 

a     Defl.    T.    23  °C.        Christy. 

— 

b     Zero    T.    18  CC.        Chr: 
C     Defl.    T.    18  °C.       Chr 
d     Zero    T.    18    C.       Chn 
C     Defl.    T.    19  °C.       Chr 
f      Zero.  T.    25°  C.    v.Oett 
g     Zero.  T.    25  °  C.    v.Oett 

X—    log  M 

y  «=  TT  =  0.058  log  ?/p    V 

sty. 
sty. 
sty. 
sty. 
ingen. 
ingen 

olts 

a 

n 

/ 

—^ 
: 

"t 

, 

f 

/ 

'fi 

/(i, 

,. 

e 

^ 

'4 

d 

— 

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f/ 

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=$?/ 

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ffi 

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4 

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— 

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e, 

J 

; 

k 

f 

/ 

b 

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j  / 

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d 

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c 

'/ 

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/ 

j/ 

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/ 

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/ 

a 

^ 

T 

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>•* 

e- 

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^ 

/ 

g 

f- 

0.3 

/ 

>' 

c 

/ 

0  4 

^ 

b 

,-jp; 

r^ 

l^ 

,  

/ 

-0.5 

/ 

f. 

> 

' 

—6 


—  5 


—  4  —3        y     -2  -1 

E.M.F.  OF  METALS  IN  KCy  SOL. 

COPPER. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

cent.,  when  it  breaks  off  sharply  and  runs  along  flat  again,  just 
as  the  zinc-curve  did. 

On  plotting  the  gold-curves,  as  has  been  done  in  Fig.  12,  it 
is  evident  that  the  gold  follows  the  logarithmic  law  fairly  well 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


49 


TABLE  Y. — Electromotive  Force  of  Gold  in  KCy  Solutions. 


Curve.                    (a). 

(&). 

(c). 

«*). 

(«H). 

to). 

Note-book,  Bl,  page          71 
Date                            Oct  20  '96 

86 
Dec  12  '96 

88 
Dec  14  '96 

172 
Aug   °6  '99 

Published. 
Feb    '99 

Published. 
Feb    '99 

Method  Deflect 

Deflect 

Deflect 

Zero 

Zero 

Zero 

Resis.  ohms  100,000 

50,000 

50,000 

Temperature  23°  C. 
Observer  Christy 

22-  C. 
Christy 

22-  C. 
Christy 

20°  C. 
Christy 

25°  C. 
Von  Oet- 

25°  C. 
Von  Oet- 

EMF  Volts 

Volts 

Volts 

Volts 

tingen. 

Volts 

tingen. 
Volts 

<N.  E.  =  —  0.560)  
Concent'n.  KCy  : 
6.4M   

+0468 

3  2M 

_l_04%>0 

1.6M  ... 

.  •• 

+0357 

\r 

+0  366 

_l_0  334 

+0  336 

+0418 

+0  340 

+0  306 

M 

+0°88 

2 
M 

+0239 

4 
M                                        '  0233 

+0  176 

+0  176 

+0  264 

+0  180 

+0218 

10  '  °*ZBa 
M 

+0135 

M 

+0  093 

40                                         
M 

+0  073 

50                                       

mjf 

i  A  ACT 

in  OAK 

+0037 

+0  065 

0092 

0  056 

loo  

M 

0  099 

—  0  073 

200 
M 

0  °44 

500                                       
M   0380 

0  306 

—0348 

—  0414 

—0474 

1,000 
M 

0  436 

T 

0533 

4jyo 

0554 

5,000 

0  560 

0439 

ior 

0  581 

20,000                                  
M 

—0567 

100,000 

M 



06°° 

1,000,000 
-M  (—  H.>O)                      nw\m 

0  fiQS 

Sep  4   "99 

Deflection. 
—0.724 
—0.620 
—0.709 

as  far  as  — -  or  0.065  per  cent.  KCy.     A  considerable  fall  of 

M 

potential  occurs,  according  to  my  experiments,  between  — 

and  ,  or  0.0065  per  cent.  KCy,  indicating  an  increase  of 

1000 

osmotic  pressure,  probably  due  to  an  increasing  dissociation  of 

4 


50 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


the  potassium  aurous  cyanide.  This  point  seems  again  a  criti- 
cal point  in  the  curve,  which,  "beyond  it,  runs  off  more  flatly, 
indicating  an  approach  to  a  constant  osmotic  pressure  of  the 
gold  ions. 

Fig.  12. 


VOLTS 

X      Axis  =  log  M 
y     Axis  =  Volts 

a.     Defl.  Method  T.  =  23°  C.  Christy 
b.     Defl.  Method  T.  =22°C.  Christy 
C.     Defl.  Method  T.  =  22°C.  Christy 
d  .    Zero.  Method  T.  =  20°  C.  Christy 
e.    Zero.  Method  T.  =  25'  C.  r.Oettingen 

/ 

'*, 

/ 

s 

^+0.3 
-1-0.2 
+  0.1 
0 
0.1 
—  0.2 
—  0.3 
—0.4 
—0.5 
—  0.6 
—0.7 

d 

/ 

Q., 

a 

£#^ 

^ 

f 

2 

s. 

e. 

a 

/ 

7 

^&G 

C  ; 

b 

d 

b' 

^ 

£pf?cc 

c 

"  f 

*/f° 

JIV-, 

^el 

c 

b 

t 

// 

/' 

I/ 

1 

/ 

b 

// 

t 

If 

^ 

// 

1  ° 

a 

^ 

d 

^ 

b/ef/ 

^ 

/ 

/e., 

/ 

/ 

a  -, 

- 

** 

' 

b 

^* 

—  • 

d 

r^ 

b  — 

d- 

— 6 


—  5 


—2 


—  0 


—  4  _3 

GOLD  IN  KCy. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n  ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

Electromotive  Force  of  Gold  in  KCl  and  KHO. 
In  order  to  bring  out  the  effect  of  the  potassium  cyanide  in 
reducing  the  osmotic  pressure  of  the  gold  ions  in  the  solution 
(according  to  the  Nernst  theory),  I  append  the  following  ex- 
periments on  the  electromotive  force  of  gold  in  solutions  of 
potassium  chloride  and  potassium  hydrate.  These  results  are 
given  in  Table  VI.,  and  are  plotted  in  Fig.  13.  It  is  evident 
that  there  is  a  very  much  smaller  electromotive  force  in  each  of 
these  cases.  It  is  particularly  low  in  the  case  of  potassium 
chloride.  According  to  the  Nernst  theory,  the  solution-pres- 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


51 


TABLE  VI.— Electromotive  Force  of  Gold  in  KCl  and  KHO. 


Curve. 

(«). 

(6). 

Note-book  Bl,  page  

160 

159 

Date  

Aug   10  '99 

Aug   10  '99 

Kesistance,  ohms  

30  (!00 

30  000 

Temperature  

IQO  r» 

18  5°  C 

Observer  

Christy 

Phri<%tv 

EMF.  (X.  E.  —  -0.560)  

Volts 

Volts 

Solution...,  

KCl 

KHO 

Concentration  : 
M 

_n  AQJ 

A    001 

j 
If 

0  510 

-0  499 

10 
M 

-0  5°3 

-0  468 

100 
M 

0  533 

-0  486 

1.000 
M 

—0  505 

10,000 
M 

-0  526 

100,000 
M 

0  551 

1,000,000 

Fig.  13. 


VOLTS 
40.2 

+  0.1 

0 

-0.1 
-0.2 
-0.3 
-0.4 
-0.5 
-0.6 
-0.7 


3.  KCL  Defl.  T.  19°  C.  Christy 
b.  KHO.  Defl.  T.  18.5° C.  Christy 
X  =  log  M 
V  =7T=  0.058  log  F/p  Volts 


—  6  -5  —4  —3  —  2  —1 

E.M.F.  OF  GOLD  IN  KCl  AND  KHO  SOLUTIONS. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M=log.  10n;  on  the 
vertical  (y)  axis,  the  actual  volts. 


52 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


sure  of  the  gold  is  the  same  in  each  of  these  solutions ;  that  is, 
the  pressure  with  which  the  gold  tends  to  go  into  solution  is 
exactly  the  same  (at  a  given  temperature),  whether  the  gold  is 
immersed  in  either  potassium  cyanide,  potassium  chloride  or 
potassium  hydrate.  But  the  number  of  gold  ions  in  each  solu- 
tion, and  hence  the  resulting  osmotic  pressure,  is  very  different. 
According  to  this  theory,  it  is  least  in  potassium  cyanide,  much 
greater  in  potassium  hydrate,  and  greatest  of  all  in  potassium 
chloride.  Consequently,  the  EMF  varies  inversely  as  p,  ac- 

p 
cording  to  the  ratio  log.  — . 

P 

The  curves  in  both  cases  run  rather  flat,  indicating  an  ap- 
proach to  a  constant  osmotic  pressure  for  high  dilutions. 

TABLE  VII. — Electromotive  Force  of  Silver  in  KCy. 


Curve. 

(«).     . 

(6). 

(c). 

(d). 

Note-book  Bl,  page...... 

71 

170 

Published. 

Published. 

Date  

Oct.  20,  '96 

Aug.  19,  '99 

Jan.,  '99 

Jan.,  '99 

Method  

Deflect, 

Zero. 

Zero. 

Zero. 

Resistance    ohms 

100  000 

Temperature                .... 

23°  C. 

19°  C 

25°  C 

25°  C 

Observer                 

Christy. 

Christy. 

Von  Oet- 

Von  Oet- 

EMF  (N.  E.  =  —0.560) 
Toncentration  KCv  : 
M 

Volts. 
+0.326 

Volts. 
+0.345 

tingen. 
Volts. 

+0.340 

tingen. 
Volts. 

+0.306 

1 
M 

-f  0.152 

+0.194 

+0  180 

+0  218 

10 
M 

-0.054 

+0.058 

-0.092 

-0.156 

100 
M 

-0.360 

-0.308 

-0.414 

-0.  474 

i,uoo 

M 

-0.417 

10.000 
M 

-0.457 

100,000 
M 

-0.498 

1,000,000 

M(-H20)... 

-0.572 

oo  \     J-i-2w; 

Fig.  14. 


VOLTS 
+  0.6 

+  0.5 
+  0.4 

+  0.2 
+  0.1 
0 
-0.1 
-0.2 
-0.3 

a  .     Defl.  T.  23UC.      Christy 
b.    Zero.  T.  19  C.     Christy 
C.    Zero.  T.  25  C.  r.Oettingen 
d.     Zero.  T.  25  C.  r.Oettingen 
X  -  log  M 
y  =  77-  =  C.068  log  %  Volte 

. 

C 

sf. 

a 

^ 

d 

: 

y 

/ 

/; 

/ 

/ 

/ 

/ 

2 

y, 

/ 

/ 

/ 

/• 

'  / 

/ 

u. 

/ 

// 

' 

, 

J. 

' 

t 

//', 

t 

S^\/ 

// 

/ 

a  V) 

-—- 

• 

' 

c1- 

r 

-0.5 
-0.6 

_b 

-r— 

,  — 

.  —  - 

—  • 

" 

d 

-i 

\ 

—  x         -6 


-5 


-4  -3  -2  -1 

E.M.F.  OF  SILVER  IN  KCy. 

On  the  horizontal  (x.)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n  ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

TABLE  VIII. — Electromotive  Force  of  Lead  in  KCy. 


Curve.                                     (a). 

(6). 

(c). 

Vote-book  Bl.  page  68 

183 

Published. 

Date                .  Oct.  20,  '99 

Sep.  2,  '99 

February,  '99 

Method               !       Deflect. 

Zero. 

Zero. 

Resistance  ohms                      i        ]  00,  000 

Temperature  j         23°  C. 

18°  C. 

25°  C. 

Observer  Christv. 

Christv. 

Von  Oettingen. 

EMF  (X.  E.  =—0.56)  „          Volts. 
Concentration  KCy  : 

M...                                                  +0.125 

Volts.* 
+0.200 

Volts. 
+  0.1  64  const. 

1 

M                                                     +O.OCO 

+0.158 

+0.128      " 

10 
M                                                    +0.006 

+  0.112 

+0.120      " 

100 
M 

+  0.070 

+0.120      " 

1,000 
M 

+0.046 

10,000 
M 

+0.040 

100,000 
M 

+0.040 

1,000,000"" 
M  (—  H,O)                                        

+0.040 

oo  I     *HVJ— 

Fig.  15. 


1 

VOL 
+0.4 
+  0.3 
+0.2 
+  0.1 
0 
-0.1 
—0.2 
0  3 

b 

— 

-  —  - 

— 

c 

C 

-^- 

—*- 

— 

-_^— 

"*" 

^ 

a 

b 

__ 

_  _ 

'  —  — 





.  

-  —  — 

— 

—  • 

—  •  — 

a 

---" 

,—  - 

—  - 

* 

--^ 

b 

a.     Defl.  T.  23JC.       Christy 

b. 

c. 

X 

Zero.  T.  18°  C.     Christy 
Zero.  T.  25  C.  r.Oettingen 
"log  M 

y  =77=0.058  log   %    Volts 

—  0.5 
—  0.6 

—  *                 —6                —5               —4                —3               —2                —  1 

E.M.F.  OF  LEAD  IN  KCy  SOLUTION. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10a ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

TABLE  IX. — Electromotive  Force  of  Mercury  in  KCy. 


Curve.                          (a). 

(6). 

(c). 

(d). 

(e). 

Note-book  Bl,  page... 
Date  

G7 
Oct.  19,  '96 
Deflect. 
100,000 
23°  C. 
Christy. 

Volts. 

+0.091 
+0.010 
+0.115 

184 
Sep.  2,  '99 
Zero. 

18°"6. 

Christy. 

Volts. 

+0.154 
+0.047 
0  04^ 

184 
Sep.   2,  '99 
Deflect. 
100,0  .0 
18°  C. 
Christy. 

Volts. 

+0.032 
-0.073 
-0.176 
-0.309 
-0.545 
-0.594 
-0.634 
-0.640 

Published. 
Feb.,  '99 
Zero. 

25°  "6. 
Von  Oet- 
tingen. 

Volts. 

+0.162 
+0.008 
-0.056 

Published. 
Feb.,  '99 
Zero. 

25°  C, 
Von  Oet- 

tingen. 
Volts. 

+0.200 
+0.024 

Method  

Resistance,  ohms  
Te  m  pe  rat  u  re 

Observer 

EMF(N.  E.  =-0.560) 
Concentration  KCy  : 

M 

1 
M 

10 
M 

100 
M 

-0.193 
-0.560 
-0.664 
-0.705 
-0.735 

1,(JOO 
M 

10,000 
M 

100,000 
M 



1,000,000'" 
M(-H20).. 

The  normal  electrode,  checked  on  ~  KC1,  showed  -0.560,  as  it  should  do. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


OO 


As  a  check  on  the  foregoing  results,  I  am  able  to  quote  the 
observations  of  an  independent  observer,  Brandenberg.*  He 
conducted  a  number  of  experiments  with  mercury  in  various 
depolarizing  solutions.  Instead,  however,  of  using  Ostwald's 
normal  electrode,  he  used  as  one  electrode  mercury  covered  with 
sulphate  of  mercury  (instead  of  the  chloride  used  in  Ostwald's). 
This  electrode  was  then  connected,  by  means  of  a  siphon  con- 
Fig.  16. 


VOLTS 
+  0.4 
+  0.3 
+  0.2 
+  0.1 
0 
-0.1 

3.     Defl.   T.  23°C.     Christy 
b.     Zero  T.  18°C.     Christy 
C.     Defl.    T.  18°C.    Christy 
d  .      Zero.  T.  25°C.  u.Oettingen 
6.     Zero.  T.  25"C.  f.Oetticgen 
X  =  log  M 
y  =  7T  =  C.058  log  %  Volts 

e 

/ 

d 

/ 

Is 

+^ 

b 

-? 

'/ 

^ 

<* 

^ 

^ 

^d 

-c- 

£ 

c 

b 

^ 

^ 

"X 

^^ 

/ 

* 

/ 

'dfe, 

/ 

/ 

^ 

/ 

af 

^ 

^ 

b 

/ 

/ 

** 

/ 

/ 

-0.3 
-0.4 

O  ^ 

1 

/ 

ifs 

/ 

/ 

'  / 

// 

/ 

I 

I/ 

O 

J 

-0.6 

^. 

^ 

•^~~ 

"^ 

b 

c 

.  - 

^-^- 

—  ' 

^ 

b- 

b 

0  8 

-s 


-5  -4  -3  -2  -1 

E.M.F.  OF  MERCURY  IN  KCy  SOLUTION. 
On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M 
vertical  (y)  axis,  the  actual  volts. 


log.  10°  ;  on  the 


taining  a  neutral  salt  in  solution,  with  a  vessel  containing  mer- 
cury covered  with  the  various  solutions  to  be  experimented  on. 
The  solutions  he  experimented  on  to  find  their  ion  destroying- 
power,  or  their  power  to  form  complex  ions  with  mercury,  were : 
potassium  sulphide,  potassium  cyanide,  potassium  sulphocyan- 
ate,  sodium  hyposulphite,  potassium  ferrocyanide  and  potassium 


Zeitschriftfiir  Physikalische  Chemie,  xi.,  p.  570,  etc. 


56 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


chloride.  As  he  did  not  use  the  same  strengths  that  I  have 
found  most  convenient,  I  have  had  to  plot  his  results,  reduce 
them  to  zero  potential,*  and  interpolate  the  results  for  the 
strengths  I  have  used.  The  results  so  obtained  are  compared 

Fig.  17. 


VOLTS 
+  0.3 
+  0.2 
+  0.1 
0 
—  0.1 
—0.2 
-0.3 
—0.4 
—0.5 
-0.6 
—  0.7 
—0.8 
-0.9 
-1.0 

a.      Zero  Method  Hg  with  KOy  (Christy) 
b-C.  Zero  Method  Hg  with  KCy  (v.Oettingen) 
X  =  log  M 
y   =7T  =   0.058  log  F/p  Volts 

A' 

ft 

^ 

c 

/ 

X 

b 

/ 

-"' 

a 

/ 

a 

^ 

^>; 

' 

,  -? 

2 

<: 

a 

^ 

? 

'"  ' 

bj 

^x 

^ 

^ 

7 

. 

/ 

J 

/ 

s 

/ 

V 

/ 

x 

a 

/ 

£ 

/ 

/ 

p 

/' 

a 

^ 

,•>> 

-^ 

^^* 

-4 

/ 

Of 

// 

$ 

i 

^ 

/ 

^ 

<P\ 

// 

v 

^ 

Z* 

/ 

u 

/^ 

/ 

I 

^ 

/ 

fL. 

^ 

a 

* 

'/I 

^ 

/, 

y 

/i 

_-  —  - 

.  —  — 

_^—  ' 

K* 

fy 

j. 

-- 

_-  v- 

~a 

// 

/ 
/ 

^ 

/ 

)  —  ' 

— 

—  v« 

a. 

! 

J2< 

>3( 

7, 

^ 

'/' 

/* 

1 

Of 

s.c: 

s 

KC 

y  y^\ 

/ 

K 

•1 

X 

•^ 

Ko 

st 

—  ' 

_--- 

KC 

a 

—6 


—  5 


—2 


-4  —3 

QUICKSILVER 

Mercury    Depolarizer/HgSO4/Hg. 

The  Results  of  Erandenberg,  (Z.f.Ph.Ch.xi.570.  &c.) 

Tlotted,  Interpolated,  Replotted  and  Reduced  to  0  Potential,  by  S.B.Christy. 
To  which  are  added,  for  comparison,  curves  with  Normal  Electrodes. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n ;  on  the 
vertical  (y)  axis,  the  actual  volts. 

with  the  results  obtained  by  Professor  Oettingen  and  myself 
with  the  normal  electrode.  The  results  obtained  by  us  for  po- 
tassium cyanide  are  higher  than  Brandenberg's,  but  show  the 
curves  to  be  of  the  same  general  nature.  They  are  shown  in 
Fig.  17. 

*  On  the  supposition  that  =-.£.•  has  a  potential  of  —  0.93  instead  of  —  0.560 


TABLE  X. — Electromotive  Force  of  Iron  in  KCy. 


Curve. 

(a). 

(6). 

(c). 

(d). 

(e). 

Note-book  Bl,  page... 
Date 

68 
Oct  21  '93 

182 
Sep    1    '99 

180 

Spn     1     'QQ 

Published. 
FoK     'QQ 

Published. 

PTaK       >OO 

Method. 

Deflect 

Deflect 

Zprn 

7f»rr» 

reo.,    yy 

Resistance,  ohms  

100,000 

100,000 

z/ero. 

Temperature  

21°  C. 

19°  C 

19°  C 

2^°  r 

oco    pi 

Observer  

Christy. 

Christy 

Christy 

Von  OP!  . 

EMF(N.  E.  =-0.56) 
Concentration  KCy  : 
M 

Volts. 
-0.169 

Volts. 
-0  028 

Volts. 
-0  124 

tingen. 
Volts. 

-4-0  5fi 

tinpen. 
Volts. 

-A  1  .Ifi 

1 
M 

-0.236 

-0  082 

0  124 

-i-O  34 

-A  19ft 

10 
M 

-0.236 

-0.116 

-0.124 

-f  0  054 

4-0  0°<> 

100 
M 

0  131 

-0  124 

0008 

I    A   f)Zl\ 

1,000 
M 

--0.146 

-0.124 

10,000 
M 

-0.160 

-0.184 

100,000 
M 

-0.160 

-0.104 

1,000,000 
M  (—  HO) 

-0.206 

-0.104 

oo  V      *VJ  

Fig.  18. 


VOLTS 

a     Defl.  T.  21  °C.  Christy 
b     Defl.  T.  19  °  C.  Christy 
C     Zero.  T.  19  °C.  Christy 
d      Zero.  T.  25°  C.  r.Oettingen 
6      Zero.  T.  25°  C.  f.Oettingen 
X  =  log  M 
y  =,  77  —  O.OC8  log  f/p   Tolts 

+  0.4 
+  0.3 
+  0.2 
+  0.1 

0 
—  0.1 

—  0.2 
0.3 

e 

d 

,,- 

^»-* 

— 

fc 

—  <. 

>-—  ' 

d 

f\ 

^ 

0 

0 

0 

E 

•^ 

b 

b~ 

*^* 

^^ 

X" 

a 

a 

-- 

•** 

-0.4 
0  5 

: 

—  0.6 

—  00  — I 


—  5  —4  —3  —2  —1 

E.M.F.  OF  IRON  IN  KCy  SOLUTION. 

On  the  horizontal  (x)  axis  are  laid  off  the  values  of  log.  M  =  log.  10n ;  on  the 
vertical  (y)  axis,  the  actual  volts. 


58 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


In  order  to  bring  out  more  clearly  the  nature  of  the  relations 
existing  between  EMP  of  the  different  metals,  I  have  combined, 
from  the  plotted  curves  of  each  metal,  what  appear  to  be  the 
most  probable  values  for  each  metal.  These  results  are  con- 
tained in  Table  XI. 

TABLE  XI. — Electromotive  Force  of  Metals  in  Cyanide  of 
Po  tassium  So  IK  tion . 


CONCENTRATION. 

COMBINATION  OF  MOST  PROBABLE  VALUES. 

M  =  10n 

Log.  10n 

Zinc. 

Copper. 

Gold. 

Silver. 

Lead. 

Mercury. 

Iron. 

M  =  10   ° 

0 

+0.945 

+0.930 

+0.420 

+0.340 

+0.200 

+0.150 

—0.030 

M  =  10-1 

—1 

+0.870 

+  0.680 

+0.205 

4-0.195 

+0.160 

+0.050 

—0.090 

M  -  10-2 

—2 

+0.775 

+0.430 

+0.090 

+0.055 

+0.110 

+0.040 

—0.120 

M  =  10"3 

—3 

+0.415 

—0.050 

—0.340 

—0.310 

+0.070 

—0.190 

—0.130 

M  -  10—  * 

—4 

+  0.385 

-0.250 

—0  450 

—0  420 

+  0.050 

—0.590 

-0.140 

M  =  10~5 

—5 

+  0.355 

—0.270 

—0.565 

—  0.460 

+0.040    !     —0.600 

—0.150 

M=10-6 

—  r> 

-f  0.330 

—0.280 

—  0.620 

—0.495 

40.040 

—  (1.635 

—0.160 

Dist.  water.. 

—  <x 

-Ki.280 

—0.320 

—0.720 

—0.570 

+0.040 

—  0.64  J 

—0.200 

These  results  have  been  plotted  in  Fig.  19.  These  curves  all 
show  critical  points  at  either  log.  M  =  —  2,  —  3  or —  4.  Most 
of  them  show  the  greatest  amount  of  inflection  at  log.  M  = 
—  3.  In  fact,  most  of  them  seem  to  change  in  character  at  this 
point.  According  to  the  Nernst-Ostwald  theory,  this  would  be 

M 

explained  by  the  assumption  that  below  say KCy,  the  dis- 
sociation of  the  complex  ion  containing  the  metal  in  point  is 
practically  complete,  so  that  the  osmotic  pressure  p  of  the  given 
metallic  ions  in  the  dilute  solutions  becomes  practically  constant 

p 
below  this  point,  so  that  as  the  ratio  —  is  nearly  constant,  so  its 

logarithm,  and  hence  the  voltage,  becomes  also  nearly  constant? 
as  is  shown  in  the  figure. 

The  curves  for  lead  and  iron  are  very  remarkable;  at  first 
quite  low,  they  maintain  themselves  at  a  higher  level  than  either 
of  the  other  metals  except  zinc.  This  is  explainable  on  the 
supposition  that  the  values  of  P  for  lead  and  iron  are  for  these 
metals  rather  low,  but  that  the  values  of  p  reach  a  nearly  con- 
stant value  sooner  than  for  the  other  metals,  so  that  the  result- 
ing curves  flatten  earlier. 

These  curves  also  show  a  number  of  remarkable  crossings. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


Copper,  which  starts  at  a  voltage  slightly  less  than  that  of 
zinc,  rapidly  falls  off,  crosses  the  curve  of  lead  a  little  below 
log.  M  =  —  2.5,  and  that  of  iron  a  little  before  log.  M  =  -  3.5, 
and  then  remains  permanently  below  these  metals.  The  gold- 


Fig.  19 


fCLTS 
+1.0 


+  0.9 


00 


_4  -3  -2  -1  0     log.  M. 

ELECTROMOTIVE  FORCE  OF  METALS 

IN  POTASSIUM  CYANIDE. 
Combination  of  Most  Probable  Values. 

curve  crosses  the  curves  of  mercury,  silver  and  iron  at  just  about 

log.  M  = 2.5.     Gold  and  silver  both  cross  mercury  again  at 

about  log.  M  ==  —  3.5.  Gold  finally  crosses  mercury  again  at  a 
point  beyond  log.  M  =  —  6,  and  remains  permanently  below  it 
after  that. 


60 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


It  will  be  observed  that  the  metals  change  their  sequence  from 
that  of  zinc,  copper,  gold,  silver,  lead,  mercury,  iron,  which 

they  possess  in  a ,  or  6.5  per  cent.  KCy  solution,  to  the  order 

zinc,  lead,  iron,  copper,  silver,  mercury,  gold,  in  distilled  water, 
which  is  the  usual  electrochemical  series  in  acid  solutions  quoted 
by  Wilson  except  that  iron  is  placed  above  lead.  The  deter- 
mination of  iron  in  my  experiments  was  not  entirely  satisfac- 
tory, by  reason,  apparently,  of  the  formation  of  films ;  and  the 
results  are  probably  too  low.  Water,  also,  appears  to  act  like 
a  weak  alkali. 

All  the  metals  show  a  critical  point  somewhere  between 
log.  M  =  — 3  and  — 4,  at  which  dilution  they  seem  to  change 
from  the  voltage  due  to  the  cyanide  solution  to  that  which  they 
ordinarily  possess. 

From  a  study  of  these  curves  there  seems  to  be  little  support 
for  the  so-called  "  selective  affinity  "  of  dilute  cyanide  solutions 
for  gold  and  silver,  except  in  the  case  of  copper  down  to 
log.  M  =  — 4,  or  0.00065  per  cent.  KCy..  In  the  case  of  zinc, 
lead,  iron  and  mercury  the  strong  solutions  give  a  better  relative 
voltage  in  favor  of  the  gold  than  do  the  dilute  cyanide  solutions. 
But  in  the  case  of  copper,  there  seems  to  be  a  distinct  advantage 
in  favor  of  the  gold  in  dilute  solutions  down  to  0.00065  per 
cent.  Then  the  curves  widen  again.  These  facts  will  appear 
from  the  following  table  taken  from  the  figure : 

TABLE  XII. — Differences  in  Electromotive   Force   Between   Gold 
and  Copper  in  Potassium  Cyanide  Solutions. 


Lo?r.  M  =  Log.  10". 

Value  of  10n. 

KCy. 

Difference  Between 
Gold  and  Copper. 

o 

1  -i-1 

Per  cent. 
6  5 

Volts. 
0  51 

1  —10 

0  65 

0  42 

_2  

1  —  100 

0.065 

0.32 

-3  

l_i-  1,000 

0.0065 

0.30 

-4  

l-r-10,000 

0.00065 

0.20 

-5 

1  -i.  1  00  000 

0  000065 

0  30 

-6 

1  _L.  1   QUO  000 

0  OOOOOb'5 

0  34 

—  oo     .       . 

1  —  Infinity  (H2O) 

0 

0  40 

It  should  be  remarked  that  if  we  had  an  independent  method 
of  determining  the  number  of  metallic  ions  in  cyanide  solu- 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  61 

tions,  and  were  thus  able  to  plot  the  EMF  in  terms  of  the  actual 
ionic  concentration  instead  of  the  molecular  concentration,  we 
should  probably  reach  a  more  perfect  agreement  with  the 
logarithmic  law  than  in  the  curves  here  shown.  Nevertheless, 
even  as  it  is,  a  general  agreement  is  certainly  evident. 

Relation  Between  the  Strength  of  Cyanide  Solutions  and  Their 
Dissolving  Power. 

It  has  already  been  shown  by  Maclaurin,*  that  the  dissolv- 
ing power  of  a  cyanide  solution  saturated  with  oxygen  increases 
with  its  strength  until  a  strength  of  5  or  10  per  cent,  is  reached, 
and  diminishes  again  as  the  strength  in  cyanide  increases  beyond 
that  point.  But,  so  far  as  I  am  aware,  no  one  has  proposed  the 
question  :  u  At  what  point  of  dilution  does  the  cyanide  solution 
cease  to  act  on  the  gold  ?" 

According  to  the  Nernst  theory,  gold  should  cease  to  dissolve 
in  cyanide  solutions,  provided  no  force  acts  except  its  own  solution- 
pressure,  at  the  point  at  which  its  electromotive  force  is  zero,  for 
then  its  solution-pressure  will  be  just  balanced  by  the  osmotic 
pressure  of  the  ions  already  in  solution.  At  this  point  (pro- 
vided no  other  force  acts)  the  solution  of  the  gold  should  cease. 

It  seemed  interesting  to  ascertain  if  there  were  such  a  point. 
In  order  to  do  so,  it  was  necessary  to  expose  the  gold  to  the 
cyanide  solution,  in  the  presence  of  air,  under  circumstances 
most  favorable  for  rapid  solution.  Hence  I  devised  a  rotating 
apparatus,  consisting  of  three  pairs  of  rollers,  driven  by  a  small 
Pelton  water-motor,  on  which  a  couple  of  2J-liter  bottles,  such 
as  are  used  for  holding  nitric  acid,  could  be  laid  and  rotated 
about  their  long  axes.  The  number  of  revolutions  of  the  middle 
axis  being  recorded,  the  distance  traveled  was  known.  This 
precaution  was  taken  to  be  able  to  allow  for  the  irregularities 
of  the  motor. 

Standard  strips  of  fine  gold  were  prepared  by  repeated  pre- 
cipitation with  sulphurous  acid  from  a  diluted  chloride  solu- 
tion. These  were  rolled  out  thin  and  cut  to  a  standard  size  of 
2  in.  by  J  in.  They  weighed  from  250  to  330  mg.,  according  to 
their  thickness.  The  strips  were  boiled  in  sulphuric  and  muri- 
atic acids,  washed,  and  ignited  before  use.  The  first  set  of  ex- 
periments was  undertaken  with  2  liters  of  solution  and  \  liter 

*  Journ.  Chan.  Soc.,  63,  p.  731. 


62  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

of  air,  the  bottles  being  stoppered.  The  weighed  gold  strips 
were  then  added;  the  bottles  were  rotated  for  24  hours;  and 
the  strips  were  then  washed  and  dried  and  weighed  again.  The 
number  of  rotations  made  in  24  hours  ranged  from  4000  to 
24,000,  and  as  the  interior  diameter  of  the  bottles  was  4J  in., 
the  distance  traveled  in  this  time  by  the  gold  strip  was  from 
one  to  six  miles.  It  was  found  impossible  to  get  a  uniform  ro- 
tation-rate, owing  to  constant  changes  in  the  water  supply.  But 
so  long  as  the  solution  was  kept  gently  agitated  these  variations 
did  not  seem  to  have  any  appreciable  effect  on  the  result.* 

Table  XIII.  shows  the  results  of  these  experiments.  The 
first  pair  were  undertaken  with  distilled  water,  to  see  if  there 
was  any  loss  due  to  erosion.  The  apparent  loss  of  0.01  mg. 
was  almost  at  the  limit  of  accuracy  of  the  balance,  but  seemed 
to  show  the  possibility  of  a  slight  loss  due  to  that  cause.  It 

will  be  observed  that  up  to  -— —  or  0.00325  per  cent,  the  gold- 
loss  is  merely  nominal,  never  more  than  0.29  mg.,  often  zero  ; 
and  the  results  vary  in  the  most  irregular  manner.  No.  18, 

with  —    -   or  0.00065  per  cent.,  gave  a  loss  of  zero,  and  No. 

20,  with  _         or  0.0016  per  cent,,  only  0.08  mg.     It  is  believed 

M 

that  these  small  losses  below         -  were  chiefly  mechanical.    It 

4000 

was  noted  that  while  most  of  the  bottles  used  were  perfect!}7 
smooth  inside,  some  seemed  to  have  small  sharp  grains  of  sand, 
or  slivers  of  glass,  projecting  above  the  smooth  inner  surface. 
In  many  cases  it  was  impossible  to  detect  these  without  break- 
ing the  bottles.  The  loss  in  No  12,  which  was  not  rotated, 
cannot  be  set  down  to  this  cause.  The  explanation  in  this 
case,  and  perhaps  in  some  others,  may  have  been  an  imperfect 
mixing  of  the  solution.  The  solutions  were  made  up  by  add- 
ing the  proper  volume  of  strong  solution  to  the  proper  amount 
of  distilled  water.  In  case  the  mixture  of  the  solutions  was 
not  thoroughly  made  before  the  gold  strip  was  added,  the  gold 

*  In  making  these  solubility-experiments,  I  was  aided  by  my  former  assistant, 
now  Assistant  Professor,  E.  A.  Hersam.  I  wish  also  to  acknowledge  the  aid  of 
my  present  assistant,  Mr.  Geo.  E.  Young,  in  the  preparation  of  the  standard 
solutions  used  in  these  experiments,  and  of  the  illustrations. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


63 


TABLE  XIII. — Solubility  of  Gold  in  Cyanide  of  Varying  Strength. 

In  twenty-four  hours.  Gold  strips,  standard  size.  Fine  gold,  2  in.  x  J  in. 
Weight,  250  to  330  mg.  2£  liter  bottles,  4£  in.  diameter,  making  4000  to  24,000 
revolutions  in  twenty-four  hours,  and  containing  2  liters  cyanide  solution  and 

liter  air. 


No. 

Strength  of  Cyanide. 

KCy. 

Per  Cent. 

Revolu- 
tions in 
24  Hours. 

Loss  Gold 
in  24  Hrs., 
Milligrms. 

Remarks. 

1 
2 

M       (H.O'l 

24,461 
13,595 

0.0  1 
0.01 

New  strip. 

x        (n.:>\J).... 
M  —  (H.7O) 

3 

4 
5 
6 
•  7 
8 
9 
10 
11 
12 
13 
14 
15 

16 
17 
18 

M 

0.000065 
0.000065 
0.000109 
0.00013 
O.C0013 
0.00016 
0.00016 
0.00016 
0.000216 
0.000216 
0.000216 
0.000325 
0.000325 
0.000325 
0.00065 
0.00065 

15,403 
10,344 
23,750 
14,430 
11,315 
7,920 
8,490 
10,180 
14,850 

/Not 
\  rotated. 

8,030 
6,490 
17,746 
17,746 
9,780 
9,780 

0.01 
0.008 
0.00 
0.00 
0.06 
0.02 
0.11 
0.19 
0.02 
0.11 
0.29 
0.20 
0.01 
0.025 
0.08 
0.00 

f  g  of  0.2  milligramme,  the 
loss  in  46  hours. 

ft  of  loss  in  45  hours. 
Same  as  above. 
Treated  24  hours. 
Duplicate  of  No.  17. 

HH),000 
M 

100,<  "00 

60,000 
M 

50,0,JO 
M 

50,000 
M 

40,000 

«g» 

40.1100 
M 

30.000 
M 

30,0(H) 
M 

SO.tiOO 
M 

20,(KK) 
M 

20,000 
M 

20,000 
M 

10.000 
M 

10,000 

Total  loss  Nos.  3  to  18  incli 

1.133 

Average  loss,  0.07  mg. 

19 
20 

M                        

0.0016 
0.0016 

14,423 
14,423 

0.26] 
0.08  J 

Mean  loss,  0.17  mg. 

401  K) 
M                         

4000 

21 
22 
23 
24 
25 
26 
27 

M 

0.00325 
0.0065 
O.OOC5 
0.013 
0.013 
0.065 
0.065 

14,423 
4,260 
4,260 
5,790 
5,790 
5.270 
5,270 

9.68 
24.86) 
21.21  ) 
81.74] 
87.48  ) 
143.64  1 
150.18  J 

Mean  of  two,  23.03  mg. 
Mean  of  two,  84.60  mg. 
Mean  of  two,  146.91  mg. 

2000 
M 

1000 

M 

1000 
M 

5<JO 
M 

500 
M                       

i  UK) 
M                           

100 

64  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

would  at  first  lie  in  a  layer  of  stronger  solution  that  might 
have  a  slight   solvent  effect  until  the  dilution  was   effected  by 

M 

rotation.    The  fact  that  the  mean  loss  up  to  •  is  only  0.07 

M 

mg.  ;  that  even  at  —  -  (Exp't  No.  18),  no  loss,  and  in  No. 

J.  VJ«  \J  \)  \J 

20  a  loss  of  only  0.08  mg.  was  obtained,  renders  it  extremely 

M 

probable  that  the  solution-loss  up  to      -TTTTA  or  0.00065  per  cent. 


is  absolutely  nil.  Thus  much  is  certain  :  these  experiments  dem- 
onstrate that  for  all  practical  purposes  the  cyanide  of  potassium 
solution  ceases  to  act  at  a  strength  below  0.001  per  cent. 

M 

Going  to  higher  strengths  we  find  a  sudden  jump  at 


here  the  loss  has  risen  to  9.68  mg.,  and  beyond  this  it  rapidly 

M 

increases;  the  strips  in  the  —  -  or  0.065  per  cent,  solution  being 

eaten  through  in  24  hours. 

The  next  set  of  experiments  was  devised  to  show  the  effect  of 
a  smaller  volume  of  cyanide  solution,  and  an  unlimited  supply 
of  air.  The  same  bottles  as  before  were  used,  but  they  con- 
tained only  500  cc*  of  solution,  and  were  left  open  to  the  air  so 
that  the  latter  was  free  to  enter.  The  results,  as  shown  in 
Table  XIV.,  are  in  general  the  same  as  before.  ~No  appre- 

M 

ciable  loss  occurs  up  to  ^r^,  but  at  that  point,  and  for  greater 


strengths,  the  loss  rapidly  increases,  finally  rising  a  little  higher 
than  before.  In  Experiments  E~os  4  and  6  the  entire  solution 
was  filtered  and  the  washed  filter  was  scorified  and  cupelled. 
In  No.  4,  where  the  gold-loss  was  0.07,  none  was  found.  In 
No.  6  (the  loss  being  0.23  mg.),  0.02  mg.  of  abraded  gold  was 
found.  Whether  the  rest  was  fine  enough  to  pass  the  filter,  or 
was  dissolved  before  the  strong  solution  was  diluted,  is  a  con- 
jecture. 

The  next  experiments  were  made  without  agitation  and  in 
the  following  manner  :  The  gold  strips  were  suspended  in 
perforated  glass  tubes  just  below  the  surface  of  the  solution,  so 
that  although  the  solution  was  at  rest,  circulation  by  convec- 
tion was  possible.  The  volume  of  the  solution  was  250  cc. 
The  time  of  action  was  in  each  case  48  hours.  The  losses  are 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


65 


TABLE  XIV. — Solubility  of  Gold  in  Potassium  Cyanide  of  Varying 
Strength  in  Ticevty-four  Hours. 

Standard  fine  gold  strips,  2  in.  X  }  in.  Weight,  250  to  330  rag.  2£ -liter 
bottles,  4£  in.  diameter,  making  4000  to  24,000  revolutions  in  twenty-four  hours. 
Half  a  liter  cyanide  solution,  2  liters  air.  Freely  open  to  air. 


No. 

Strength  of  Cyanide. 

KCy. 
Per  Cent. 

Revolutions  in 
24  Hours. 

Gold  Loss  in 

'24  Hours, 
Milligrammes. 

1 

M  (—HO) 

5110 

0  01 

X,     V         **-2^)  

^  C  —  HO) 

5110 

000 

~M    ' 

0.0005 

8440 

0.43  (') 

3 

12,800 
M 

0.0005 

6COO 

0.07* 

4 

12,800 
M 

0.001 

8440 

0.19 

5 

6400 
M 

0.001 

6600 

0.23f 

6 

6400 
M 

0.0016 

6790 

0.16 

7 

4000 
M 

0.002 

5450 

0.44 

8 

3200 
M 

0.00325 

6790 

1.77 

9 

20UO 
M 

0.004 

5450 

4.29 

10 

1600 
M 

0.008 

5540 

48.43 

1J 

800 
M 

0.016 

^ 
/5540 

t  , 

74.96 

\'l 

400 
M 

0.0325 

28,230 

150.54 

13 

200 
M 

0.065 

28,230 

168.12 

14 

100 

given  in  Table  XV. ;  they  are  somewhat  smaller  than,  before, 
and  again  negligible  below  In  this  case  ~^n^  was  not 

J-\J\JU  £  V'  M  M 

determined.     The  advantage  of  the  position  near  the  surface 
near  the  air  is  well  shown  in  comparing  Nos.  5  and  8.     When 

the  gold  was  suspended  near  the  surface  of  a  -       solution,  the 
loss  was  21.  44  mg.  in  48  hours,  or  nearly  half  a  milligramme 

*  In  order  to  see  whether  or  not  these  losses  might  not  be  due  in  part,  or 
wholly  due  to  abrasion,  the  solution  was  filtered  and  the  filter  scorified  and 
cupelled.  No  gold  was  found  in  the  filter. 

f  This  solution  was  also  treated  as  above,  and  0.02  mg.  of  abraded  gold  was 
found. 

5 


66 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


per  hour :  when  the  strip  was  put  at  the  bottom  of  the  same 
volume  of  a  similar  solution,  the  total  loss  in  the  same  time  was 
only  8.70  mg.,  or  hardly  one-third  as  much. 

TABLE  XV. — Solubility  of  Gold  in  Potassium  Cyanide  of  Varying 
Strength  in  Forty-eight  Hours.     At  Rest. 

Standard  fine  gold  strips,  2  in.  X  \  in.  "Weight,  250  to  330  mg.  Suspended 
in  open  glass  tubes,  near  surface,  of  250  c.c.  Cyanide  solution  at  rest,  but  so  that 
convection  currents  were  possible. 


No. 

Strength  of  Cyanide. 

KCy. 
Per  Cent. 

Gold  Loss  in 
48  Hours, 
Milligrammes. 

M 

0.00^065 

0  00 

1 

100,000 
M 

0  C0065 

0  06 

2 

10,000 
M 

I         0.0065 

4.33 

3 

loOO 
M 

0  003 

3.86 

4 

8bO 
M 

0  016 

21  44*   " 

5 

400 
M 

00325 

36.57 

6 

•  200" 
M 

i        0065 

42  79 

7 

100 

| 

All  these  results  have  been  plotted  together  with  the  volt- 
age of  the  gold  in  curves  a,  6,  c  and  d,  in  Fig.  20.  According 
to  the  voltage-curve  the  voltage  becomes  zero  for  a  cyanide 
solution  of  0.00675  M,  or  about  0.044  per  cent.,  and  it  is  a 
curious  fact  that  this  is  very  near  the  limit  of  strength  that 
practice  has  so  far  justified.  Nevertheless,  solutions  as  low  i.s 
0.01  per  cent.,  and  even  less,  have  been  employed  in  practice; 
and  my  experiments  show  that  the  solution  acts  perceptibly 


M 

down  as  low  as  -  —  or  0.00325  per  cent.,  and  perhaps  to 


M 

4000 


2000 
or  0.0016  per  cent.  KCy. 

An  Apparent  Contradiction. — Although  it  will  be  seen  that  the 
solubility-curves  all  follow  very  closely  the  voltage-curve,  the 
fact  that  action  does  not  cease  for  the  zero  of  potential  of  gold 


*  8.  A  similar  experiment  with  same  conditions  as  to  strength  and  volume  of 
solution  as  No.  5,  only  that  the  strip  of  gold  rested  at  the  bottom  of  the  vessel, 
gave  a  gold  loss  of  only  8.7  mg. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


67 


in  KCj  solution  seems  to  contradict  the  ]N"ernst  law.  But  this 
contradiction  is  only  apparent  and  not  real.  It  is  part  of  the 
new  theory  that  (except  the  infinitesimal  amounts  necessary  to 
produce  the  state  of  static  tension)  ions  cannot  come  into  ex- 


istence or  disappear  except  in  pairs.  That  is,  for  every  ion  with 
a  positive  charge  of  electricity  there  must  be  one  with  an  equal 
negative  charge.  Hence,  when  a  positive  ion  appears,  another 
positive  ion  must  disappear,  or  else  a  negative  one  must  also 


68  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

appear  simultaneously.  (In  the  case  of  ions  with  varying  val- 
ency, an  ion  having  a  double  or  treble  valency  is,  of  course, 
equivalent  to  two  or  three  oppositely  electrified  univalent  ions.) 
For  instance,  in  my  experiments  for  determining  the  electro- 
motive force  of  gold  in  potassium  cyanide  solution  against  the 
normal  electrode,  the  couple  is  composed  of 

Gold :  KCy 
KC1,  HgCl :  Hg' 

When  the  gold  dissolves,  the  positive  gold  ions  travel  from  the 
gold  with  their  positive  charge,  and  in  order  that  the  solution 
may  continue,  a  similar  flow  of  positive  ions  must  continue 
by  means  of  potassium  ions  through  the  solution  to  the  mer- 
cury.  The  potassium  ions  finally  drive  out  some  of  the  mercury 
ions  which  precipitate  into  the  mercury  forming  the  electrode, 
at  the  same  time  giving  up  their  charge  of  positive  electricity  to 
it.  Simultaneously  there  is  a  corresponding  flow  of  negative 
ions  in  the  opposite  direction.  Thus  :  first  chlorine  and  then 
cyanogen  move  in  the  opposite  direction  to  meet  the  gold,  and 
the  latter  forms  with  the  cyanogen  the  complex  negative  ion 
( AuCyO  (— ).  Thus:  Au  (+)  +Cy  (— )  +Cy  (— )  =  (AuCy.2)  (— ). 
The  action  of  this  couple  will  go  on  so  long  as  the  electromotive 
force  of  the  combination  is  greater  than  zero,  and,  as  we  have  seen  in 
my  experiments,  long  after  the  electromotive  force  of  the  gold 
in  the  dilute  cyanide  solution  has  become  zero.  For  the  tend- 
ency of  the  mercury  ions  to  discharge  into  the  mercury  elec- 
trode can  only  affect  its  purpose  and  cause  a  current  by  the 
simultaneous  solution  of  the  gold.  That  is,  the  tendency  of 
the  positively  electrified  ions  of  mercury  to  discharge  themselves 
can  cause  the  gold  to  dissolve  long  after  its  own  electromotive 
force  has  ceased. 

•    The  Electromotive.  Force  of  the  Oxygen  of  the  Air  the  Sufficient 

Cause  of  the  Solution  of  Gold  in  Cyanide  Solutions. 
We  have  another  substance  at  hand  with  a  great  tendency  to 
form  negative  ions.  This  is  the  oxygen  of  the  air.  In  the 
presence  of  water,  the  molecule  of  oxygen,  O2,  tends  to  assume 
the  ionic  state,  combining  with  water  to  form  four  negatively 
electrified  ions,  thus :  0,  (±)  +  2H20  (±)  ==  4  (OH)  (— ).  Or, 
as  has  been  suggested  by  Traube,  when  metals  dissolve  in  the 
presence  of  oxygen,  a  molecule  of  the  latter  combines  directly 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


69 


with  two  atoms  of  potentially  nascent  hydrogen  thus :  02  -f  2H 
=  H202.  Later,  the  peroxide  of  hydrogen  dissociates  into  two 
negative  hydroxyl  ions,  which,  entering  the  solution  with  their 
negative  charges  of  electricity,  tend  to  produce  a  current  in 
the  same  direction  as  the  positively  electrified  mercury  ions  do 
when  they  leave  the  solution.  That  is,  oxygen  can  play  the 
same  part  in  causing  the  solution  of  the  gold  as  the  mercury 
ions  did  in  the  normal  electrode  ahove  cited. 

The  controlling  importance  of  an  abundant  supply  of  oxygen 
is  well  shown  by  the  curves  in  Fig.  20.  In  curve  e,  although 
there  is  only  one-fourth  as  much  cyanide  present  as  in  curve  6, 
the  amount  of  gold  dissolved  is  greater,  except  for  the  very  di- 
lute solutions.  The  evident  reason  is  that  the  aeration  is 
greater.  The  cyanide-supply  being  ample  in  both  cases,  the 
oxygen-supply  determines  the  rate  of  solubility.  For  dilute 
solutions,  the  amount  of  dissolved  oxygen  being  sufficient  in  6, 
the  greater  volume  of  cyanide  is  the  determining  factor,  and 
the  amount  dissolved  in  b  is  in  this  case  greater  than  in  c. 

Interesting  confirmation  of  these  views  is  found  in  Maclaurin's 
experiments  on  the  solubility  of  gold  in  a  solution  of  cyanide  of 
potassium  saturated  with  oxygen.*  He  conducted  two  sets  of 
experiments  with  gold  strips  in  solutions  of  different  strengths. 
The  first  set  was  left  at  rest  for  three  hours,  the  second  set 
was  agitated.  The  losses  are  given  in  the  following  table : 

TABLE  XVI. — Maclaurin's   Table  of  Losses  of  Gold  in 
Saturated  with  Oxygen. 

At  Kest  in  Solution  Saturated  with  Oxygen.     Time,  Three  Hours. 


KCy   per  cent      

1 

o 

10 

20 

30 

40 

50 

Gold  loss   ing  "1"         

8.45 

13.55 

15.40 

11.15 

8.55 

5.8 

5.05 

Agitated  for  Two  Hours  in  Solution  Saturated  with  Oxygen. 


KCy  per  cent                     .  .. 

1 

4.9 

9.4 

19.93 

29.9 

39 

47.3 

(jrold  loss   nig  i              

18.7 

47.2 

39.1 

31.4 

21.1 

14.2 

10.8 

Maclaurin  deems  the  results  in  the  second  table  more  reliable 
than  those  in  the  first.     In  both  it  will  be  seen  that  there  is  a 


*  Journal  Chemical  Society,  Ixiii.,  p.  731. 
f  Curve  A,  Fig.  21. 


Curve  B,  Fig.  21, 


70 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


rapid  increase  of  the  dissolving  power  up  to  about  5  or  10 
per  cent.  KCy;  then  it  gradually  falls  off  till  at  50  per  cent,  the 
solubility  of  the  gold  is  less  than  at  1  per  cent. 

The  importance  of  the  remarkable  relation  thus  discovered 
by  Maclaurin  has,  I  think,  never  before  been  appreciated.  Is  it 
not  a  little  remarkable  that  the  strong  cyanide  solution  should 
dissolve  less  gold  than  a  weak  one,  while  the  electromotive 
force  of  the  gold  goes  on  steadily  increasing  ? 

But  in  the  light  of  the  new  theory  the  reason  is  not  far  to 
seek,  for  at  no  time  does  the  electromotive  force  of  the  gold  rise 
high  enough  to  displace  without  external  aid  any  other  positive 
ions,  such  as  those  of  the  potassium  in  the  cyanide  or  the  hy- 
drogen in  the  water;  and  unless  this  be  done,  the  gold  ions 
cannot  continue  to  form,  nor  the  gold  to  dissolve.  For  this 
reason  (as  Maclaurin,  myself  and  others  have  shown),  in  the  ab- 
sence of  oxygen  or  some  equivalent  agency,  gold  does  not  dis- 
solve in  cyanide  solutions.  In  other  words,  unless  some  nega- 
tive ion  like  (OH)  ( — ),  (01)  ( — ),  or  (Br)  ( — )  is  added,  or  some 
other  positive  ion  as  (K)  (-f)  etc.  is  removed  by  some  external 
source  of  energy,  the  action  cannot  go  on.  Ordinarily  the  oxy- 
gen of  the  air  furnishes  this  energy;  as  we  have  seen  above, 
it  dissolves  in  the  solution  and  furnishes  the  negative  ions  nec- 
essary to  cause  the  solution  of  the  gold. 

Again,  Maclaurin  has  found  the  key  to  the  anomalous  action 
of  strong  cyanide  solutions.  It  is  in  the  fact  which  he  demon- 
strated, that  oxygen  is  less  soluble  in  strong  than  in  weak  cyanide 
solutions.  The  following  results  for  the  solubility-coefficient  of 
oxygen  in  KCy  are  plotted  from  his  curves  by  interpolation. 

TABLE  XVII.— Solubility  of  Oxygen  in  Solutions  of  KCy  at  18°  C. 

(Maclaurin). 


Strength  KCy,  percent...        1 

5 

10 

20 

30 

40 

50 

Solubility,  per  cent  0  0295 

0.0235 

0  019 

0  014 

0  0103 

Solubility,  per  cent.*  j  0.0290 

0.022 

0.018 

0.013 

0.008 

0.005 

0.003 

I  have  replotted  the  results  of  the  above  experiments  of  Mac- 
laurin so  as  to  make  them  more  comparable  with  my  own  re- 
sults. I  have  replotted  both  the  gold  losses  of  Maclaurin  and 
the  second  of  his  oxygen  solubility  coefficients  in  Fig.  21 ;  and 


Curve  C,  Fig.  21. 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


71 


GOLD  LOSS                                                                   Fig.  21. 
MGS. 

50 

x^S. 

/    * 

R 

» 

45 

J      Gold  Loss   n  KCy  saturated  with  Oxyge 
Agitation  2  hours.      (Maclaurin) 

A     Gold  LOFS  in  KCy  saturated  with  Oxyge 
At  rest  3  hours       (Maclaurin) 

C     Solubility  of  Oxygen  in  KCy  solutions. 
Per  cent  volume  x  1000      (Maclaurin) 

3      Gold  Voltage  Curve  (Christy.) 

i. 

u 

An 

;E: 

\ 

\ 

\ 

35 

\ 

B 

\ 

\ 

30  - 

\ 

1 

\ 

I 

ft 

\ 

25  - 

\\ 

\ 

3 

\\ 

\ 

\ 

> 

\ 

20  - 

\ 

1 

\ 

C 

\ 

\ 

\ 

R 

A 

s. 

\ 

15  I- 

1 

Ny 

\ 

\ 

VOLTS 

\ 

ss 

\ 

:z; 

•  1s 

0.60 

-/-- 

5 

—    — 

—  — 

••  •- 

—  — 

^ 

- 

B 

D 

10  U 

j 

i  — 

—  - 

—  ' 

ix 

— 

»-—  • 

U 

••  — 

0.50 

/ 

U 

t 

^ 

^ 

^ 

0.40 

> 

N 

> 

\ 

0.30 

5 

/ 

\ 

"-  — 

A 

x 

0.20 

V 

"^ 

C 

0.10 

0 

10  20  30 

SOLUBILITY  OF  GOLD  AND  OXYGEN  IN  KCy, 

From  experiments  of  Maclaurin.    (J.  Chem.  Soc.  1893,  pg.  724.) 
Rtplotttd,  and  Gold  Voltage  -  Curve  Added. 

I  have  also  added  the  voltage-curve  for  gold  from  iny  own  ex- 
periments. 


72  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

It  is  plain  now,  for  the  first  time,  why  there  should  be  a  maxi- 
mum solubility  somewhere  between  5  and  10  per  cent.  There 
are  two  causes  at  work  tending  to  dissolve  the  gold.  First,  the 
electromotive  force  of  the  gold  itself,  which  alone  is  insufficient 
for  the  purpose;  and  second,  that  of  the  hydroxyl  ions.  If  we 
suppose  the  latter  proportional  to  the  solubility  of  the  oxygen, 
we  see  that  the  two  forces  operating  to  cause  the  solution  of  the 
gold  tend  to  increase  in  inverse  relation.  Further,  that  the 
electromotive  force  of  the  gold  rises  very  rapidly  till  it  gets  to 
between  5  and  10  per  cent,  and  then  rises  very  slowly  after 
that,  so  that  it  has  little  effect  on  the  solubility  beyond  that 
point.  The  solubility  of  the  oxygen  (and,  as  we  have  as- 
sumed, of  the  hydroxyl  ions)  is  a  maximum  for  pure  water, 
and  sinks  as  the  gold-voltage  rises.  It  is  at  between  5  and  10 
per  cent,  that  these  two  factors  give  their  maximum  effect. 
Beyond  that  point,  the  solubility-curves  of  the  solution  for  oxy- 
gen and  for  gold  run  along  nearly  parallel. 

Neither  of  these  two  factors  alone  is  able  to  account  for  the 
maximum  point  in  the  solubility-curve.  If  the  presence  of  oxy- 
gen were  the  only  cause,  the  maximum  solubility  would  be  with 
dilute  solutions.  If  it  were  alone  due  to  the  electromotive  force 
of  the  gold,  it  would  be  greatest  in  strong  solutions.  As  both 
act  together,  the  maximum  effect  lies  between  these  extremes. 

As  far  as  I  am  aware,  this  inverse  relation  between  the  electro- 
motive force  of  gold,  and  that  of  oxygen  in  cyanide  solutions  of  • 
varying  strength  as  a  controlling  factor  in  determining  the  sol- 
ubility of  gold  in  such  solutions  has  never  been  brought  out  be- 
fore. In  a  certain  sense  it  is  a  turning-point  in  this  discussion, 
and  hence  merits  a  little  close  attention. 

The  ionizing  tendency  of  oxygen  has  been  measured  by  a 
cell  containing  a  platinum  electrode  made  absorbent  for  oxy- 
gen by  coating  it  with  platinum  sponge.*  When  this  is  im- 
mersed in  oxygen  at  atmospheric  pressure,  and  the  end  of  the 

wire  is  immersed  in  —  sulphuric  acid,  and  the  latter  is  con- 
nected with  the  normal  electrode,  the  mercury  dissolves,  and  a 
positive  current  flows  through  the  solution  from  the  mercury  to 
the  platinum  with  a  potential  of  +  0.75  volt. 

*  Le  Blanc,  Electro-chemistry,  p.  221. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  73 

This  current  moves  in  the  opposite  direction  to  that  due  to 
the  electromotive  force  of  the  mercury,  viz.:  — 0.560  volt;' 
consequently  the  electromotive  force  of  the  oxygen  at  atmos- 

M 

pheric  pressure  in  contact  with  platinum  sponge  in  — ,  sulphuric 

acid  is  equal  to  the  sum  of  these,  or  1.31  volts.  It  is  negative, 
or  — 1.31  volts,  since  negative  ions  are  produced,  and  the  solu- 
tion is  negatively  electrified  by  them. 

If  the  above  determination  is  correct,  it  follows  that  if,  in- 
stead of  the  normal  electrode  in  the  above  combination,  we 
place  a  vessel  containing  a  gold  electrode  and  a  solution  of  cy- 
anide of  potassium  so  weak  that  the  potential  of  the  gold  is  not 
merely  zero,  but  as  low  as  that  of  the  mercury,  viz. :  — 0.560, 
it  is  plain  that  a  similar  voltage  of  +  0.75  should  exist;  but  in 
this  case  the  gold  would  dissolve  instead  of  the  mercury,  and 
the  positive  current  would  flow  through  the  solution  from  the 
gold  to  the  platinum  as  before.  In  this  case  it  wrould  be,  of 

course,   necessary  to   interpose    an  1-  solution  of  K2S04,  KC1, 

or  some  other  neutral  salt,  between  H2S04  and  the  KCy,  to  pre- 
vent their  direct  action  with  each  other  from  interfering  with 
the  mere  transfer  of  electromotive  forces  at  the  end  of  the  line 
which  we  wish  to  effect. 

Now  gold  does  not  absorb  and  ionize  oxygen  as  readily  as 
platinum  does,  but  it  acts  similarly,  though  to  a  much  less  ex- 
tent. In  order  to  test  the  correctness  of  these  views,  I  took  two 
small  porcelain  cups,  B  and  0,  Fig.  22,  in  which  were  immersed 
the  two  electrodes  b  and  o.  These  were  gold  strips  held  in 
platinum-tipped  forceps,  connected  in  series  with  a  reflecting 
galvanometer  G  of  3000  ohms  resistance,  including  that  of  the 
cell,  and  a  resistance  R  of  30,000  ohms.  The  solution  in  either 
vessel  is  connected  electrically  by  the  liquid  in  the  siphon  C. 

It  is  very  difficult  to  prepare,  and  impossible  to  keep,  a  cya- 
nide solution  entirely  free  from  oxygen,  unless  it  is  hermetically 
sealed.  But  the  following  method  was  selected  as  giving  an 
approximation  to  it.  A  liter  of  distilled  water  was  boiled 
under  a  filter-pump,  and  when  most  of  the  dissolved  oxygen 
had  been  removed,  cyanide  of  potassium  was  added,  and  the 
boiling  was  continued  a  few  minutes,  to  drive  out  the  air 
absorbed  during  the  solution  of  the  cyanide.  A  cork  was  pro- 


74 


THE    ELECTROMOTIVE    FORCE    OF    METALS. 


vided  with  two  tubes  like  those  of  an  ordinary  wash-bottle  ; 
and  after  inserting  the  long  tube  below  the  surface,  a  layer  of 
paraffine  oil  was  floated  on  to  the  surface  to  exclude  the  air. 
The  tip  of  the  discharge-tube  was  kept  closed  by  a  cork  when 
not  in  use.  It  was  easy,  by  blowing  in  through  the  short  tube 
above  the  surface  of  the  oil,  to  discharge  any  required  amount 
of  the  solution  as  required,  but  of  course  each  time  this  was 
done  a  small  amount  of  air  entered  the  solution.  After  cool- 


Fig.  22 


APPARATUS  FOR  SHOWING  THE 

LOCAL  ELECTROLYTIC  ACTION 

INVOLVED  IN  THE 

SOLUTION  OF  GOLD  IN 
AERATED  CYANIDE  SOLUTION 

ing,  the  liquid  was  titrated  and  found  to  contain  0.62  per  cent. 
KCy.  A  similar  0.621  per  cent.  KCy  solution  was  prepared  and 
nearly  saturated  with  oxygen.  Through  the  galvanometer  G 
and  the  resistance  R,  a  Latimer-Clark  cell  gave  a  deflection  of 
7  scale-divisions.  In  vessel  B  were  placed  12  c.c.  of  boiled 
0.62  per  cent.  KCy  solution  and  in  0  an  equal  volume  of  0.621 
per  cent.  KCy  solution  containing  oxygen.  On  immersing  the 
gold  strips,  the  strip  in  B  became  negative,  that  is,  the  positive 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  75 

current  flowed  from  B  through  the  solution  to  O,  with  an 
KMK  =  =  -fO.02  volt.  When  the  liquid  in  both  B  and  0  was 
covered  with  paraffine  oil  to  exclude  the  air,  the  EMF  rose  to 
-f  0.108  volt.  On  gently  shaking  electrode  o,  the  EMF  rose 
to  -f  0.185  volt;  on  gently  shaking  B  it  fell  to  -fO.08  volt 
(owing  to  absorbed  oxygen).  On  cutting  out  the  30,000  ohms 
resistance,  leaving  that  of  the  galvanometer  (3000  ohms),  the 
deflection  rose  to  6.5  scale-divisions,  coming  back  again  on  in- 
serting the  resistance  E  to  0.6  scale-divisions  or  +0.12  volt. 
This  gradually  fell  to  0.2  scale-division  or  -f  0.04  volt,  where 
it  remained  for  two  hours.  At  the  end  of  that  time  the  resist- 
ance was  cut  out  and  the  deflection  rose  to  2.5  scale-divisions ; 
then,  on  shaking,^to  12  divisions;  and  then  sank  again  to 
2.7,  where  it  remained  fairly  steady  for  two  hours  longer.  At 
the  end  of  this  time,  four  hours  in  all,  the  electrodes  were  re- 
moved and  cleaned  with  gasoline  and  ether  from  the  oil  and 
solution  ;  and  it  was  found  that  the  electrodes  had  lost  weight 
as  follows : 

b  lost  1.28  mg.  o  lost  1.73  mg. 

The  solutions  contained  in  the  vessels  B  and  0  and  in  the 
siphon  C  were  also  assayed  with  the  following  results : 

B  contained  1.25  mg.,  O  contained  1.68  mg.,  and  C  contained 
0.06  mg.  of  gold. 

The  total  loss  of  the  electrodes  was  3.01  mg.,  and  that  found 
was  2.99  mg.  The  difference  of  0.02  mg.  was  probably  lost  in 
the  washings  of  the  electrodes,  which  were  not^saved. 

This  experiment,  corroborated  by  many  others,  shows  clearly 
that  the  positive  current  flows  from  the  deoxygenated  to  the 
oxygenated  cyanide,  just  as  theory  would  indicate.  The  fact 
that  more  gold  has  dissolved  in  the  oxygenated  than  in  the  de- 
oxygenated  cyanide  does  not  militate  against  the  indication  of 
the  galvanometer. 

The  solution  of  the  gold  in  the  vessel  O  is  evidently  due  to 
the  well-known  phenomenon  of  "  local  action."  The  current 
that  flows  through  the  siphon  has  to  overcome  a  resistance  of 
from  3000  to  33,000  ohms,  while  local  action  can  go  on  in  the 
vessel  0  wherever  an  OH  ( — )  ion  comes  in  contact  with  gold 
and  KCy.  Here  it  forms  a  "  short  circuit,"  and  it  completes 
itself  on  the  gold  strip  o  at  any  point  free  from  oxygen,  without 
having  to  pass  through  the  entire  external  circuit. 


76  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

It  might  be  objected  that  the  fact  that  1.73  mg.  of  gold  had 
dissolved  in  0  as  against  1.28  in  B  only  went  to  prove  that 
some  oxygen  had  been  contained  in  B,  though  less  than  in  0, 
and  that  the  solution  in  each  had  been  simply  in  proportion  to 
the  oxygen  present.  But  this  does  not  account  for  the  abso- 
lute verdict  of  the  galvanometer,  which  shows  that  the  positive 
current  flows  during  the  entire  experiment  from  strip  b  through 
the  solution  to  the  strip  o.  The  only  explanation  that  remains 
is  the  one  which  I  have  suggested.  There  is  no  doubt  that 
considerable  local  action  went  on  in  cell  0.  That  this  was  the 
case  is  also  evidenced  by  the  fact  that  the  action  was  more  uni- 
formly distributed  over  the  surface  of  6,  while  the  strip  o  was 
not  uniformly  acted  on,  but  was  eaten  into  in  a  remarkable 
manner.  These  strips,  and  particularly  some  of  those  to  be  de- 
scribed later  (with  peroxide  of  hydrogen),  were  not  corroded 
most  upon  the  edges  where  one  would  naturally  expect  it,  but 
along  vertical  lines  running  up  and  down  the  middle  of  the 
strip.  In  some  cases  they  were  eaten  through  along  these 
lines  in  such  a  manner  that  nothing  remained  but  a  thin  film 
like  gold  lace.  It  appeared  that  local  action  started  in  along 
these  lines  rather  than  at  the  edges,  owing  to  differences  of 
potential  due  to  the  distribution  of  the  oxygen,  and  that  when 
it  had  once  set  in,  it  was  able  to  maintain  itself. 

It  is  probable  that  in  all  cases  of  the  solution  of  gold  in 
aerated  cyanide  solutions  the  process,  as  in  the  above  case,  is 
one  of  local  electrolytic  action,  though,  as  it  is  impossible  in 
such  a  case  to  apply  the  galvanometer,  it  would  be  difficult  to 
prove  this  proposition  except  by  inference. 

In  all  such  experiments  it  is  important  to  be  certain  that  the 
gold  strips  are  in  the  same  physical  state,  since  the  existence 
of  microscopic  films  or  unweighable  traces  of  occluded  gas 
cause  an  appreciable  difference  of  potential  in  apparently  similar 
gold  strips.  This  is  best  tested  by  comparing  the  strips  in 
the  same  solution.  They  react  similarly  if  they  are  carefully 
cleaned  with  boiling  acid,  and  are  then  washed  with  distilled 
water  and  ignited  to  redness  side  by  side  in  the  muffle  or  over 
a  Bunsen  flame  in  a  small  porcelain  dish.  But  if  they  are 
heated  in  different  parts  of  the  same  Bunsen  flame,  they  fre- 
quently show  quite  appreciable  differences  of  potential  due  to 
occluded  gases. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  77 

The  Effect  of  Hydrogen  Peroxide. 

The  peroxide  of  hydrogen  used  was  Marchand's  medicinal, 
containing  3.3  per  cent,  of  available  peroxide,  as  determined  by 
titration  with  permanganate  of  potassium.  According  to  the 
new  theory,  the  H2O2  (±)  takes  up  from  the  gold  strip  o,  which 
becomes  positive,  two  units  of  negative  electricity  and  dis- 
sociates into  2  (OH)  (— ). 

M 

In  the  first   experiment  a  — -  KCy  solution   containing  the 

usual  amount  of  absorbed  oxygen  was  used,  and  10  c.c.  of  this 
solution  was  placed  both  in  B  and  in  O.  Gold  strips  b  and  o 
were  then  placed  in  B  and  O,  and  the  siphon  was  inserted. 
Both  strips  showed  themselves  of  the  same  potential.  The 
siphon  was  removed  and  5  c.c.  of  water  was  added  to  B  and  5 
c.c.  of  hydrogen  peroxide  to  0.  On  inserting  the  siphon  and 
the  electrodes,  b  proved  to  be  electronegative,  that  is,  the  solu- 
tion in  B  was  electropositive  by  +  0.66  volt;  in  other  words, 
the  positive  current  flowed  through  the  solution  from  6  to  o. 

Another  experiment  was  made  with  boiled  water  with  0.62 
per  cent,  KCy  that  had  been  kept  under  \  in.  of  oil  for  a  week. 
B  and  O  were  each  filled  with  10  c.c.  of  this  solution,  and  the 
gold  strips  and  siphon  were  inserted.  The  strips  proved  to  be 
of  the  same  potential.  The  siphon  was  then  removed,  and  to 
B  was  added  2  c.c.  of  distille'd  water,  and  to  O  two  c.c.  of  per- 
oxide of  hydrogen.  After  mixing,  on  replacing  the  siphon, 
the  voltage  rose  to  +0.57  volt.  That  is,  the  positive  current 
flowed  through  the  solution  from  b  to  o.  To  exclude  the  air, 
a  layer  of  paraffine  oil  about  J-in.  thick  was  floated  over  each 
solution  before  inserting  the  siphon. 

The  resistance  of  30,000  ohms  was  then  cut  out,  leaving 
only  that  of  the  galvanometer  (3000  ohms),  and  the  needle 
which  had  previously  shown  a  deflection  of  2.6  scale-divisions 
was  thrown  out  of  sight.  (The  limits  of  the  scale  used  were 
21.0  scale-divisions.)  After  being  thus  short-circuited  for  an 
hour  and  a  half,  on  throwing  in  again  the  30,000  ohms  resist- 
ance, the  voltage  of  the  combination  showed  itself  to  be  still 
in  the  same  direction,  +  0.63  volt.  The  30,000  ohms  were 
again  cut  out  and  the  combination  was  again  short-circuited 
overnight.  In  the  morning  some  bubbles  of  gas  from  the 


78  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

action  of  the  peroxide  had  collected  in  the  upper  part  of  the 
siphon,  and  had  nearly  cut  off  the  current.  But  on  removing 
and  refilling  the  siphon  the  voltage  still  showed  itself  to  be  in 
the  same  direction,  -f  0.55  volt.  The  resistance  of  30,000 
ohms  was  again  cut  out  and  that  of  the  galvanometer  only  left 
in,  and  after  5J  hours  more  the  electrodes  were  taken  out  and 
cleaned  and  weighed.  Total  time,  23  hours. 

The  strip  contained  in  B  had  lost  13.25  mg.,  while  that  in  0 
had  lost  only  9.20  mg.  Evidently ,  in  spite  of  the  local  action  that 
had  taken  place  in  the  vessel  0,  more  gold  had  dissolved  in  the  vessel 
B  in  the  absence  of  the  oxidizing  agent,  than  in  0  where  the  oxidizing 
agent  was  present. 

In  order  to  determine  how  much  of  the  loss  in  B  might  be 
due  to  dissolved  oxygen  which  had  leaked  through,  or  by,  the 
oil-cover  into  the  cyanide  solution  since  it  had  been  made,  a 
week  previously,  10  c.c.  of  the  same  solution  as  that  used  in  B 
was  placed  in  a  similar  vessel,  and  a  gold  strip  was  immersed 
in  it  half-way,  and  the  liquid  was  then  covered  with  the  par- 
affine  oil  just  as  had  been  done  in  B  and  0.  After  19J  hours 
it  had  lost  4.28  mg.  A  similar  strip  entirely  submerged  below 
solution  and  oil  lost,  in  24  hours,  2.64  mg.  These  experiments 
prove  that  some  air  had  leaked  through,  or  by,  the  oil  cover. 
It  had  been  previously  proved  that  if  a  thicker  layer  were  used, 
it  was  possible  practically  to  prevent  altogether  the  ingress  of 
oxygen  and  the  solution  of  the  gold.  In  this  case  it  was  in- 
convenient to  use  a  layer  thicker  than  J-in.  But  the  experi- 
ment also  clearly  shows  that  the  amount  of  gold  thus  dissolved 
by  absorbed  oxygen  is  so  much  less  than  that  shown  by  the  b 
strip,  that  the  solution  must  have  been  caused  by  the  electro- 
motive forces  of  the  combination  in  the  manner  I  have  explained. 

The  same  experiment  was  repeated  exactly  as  before,  except 
that  to  10  c.c.  of  0.62  per  cent.  KCy  in  B  was  added  1  c.c.  ot 
water  and  to  10  c.c.  in  0  was  added  1  c.c.  of  peroxide  of  hydro- 
gen. At  first  the  voltage  was  -f  0.652  volt,  rapidly  falling  to 
+  0.63  volt.  After  cutting  out  all  but  3000  ohms  resistance  for 
21  hours,  the  voltage,  on  adding  the  30,000  ohms,  proved  to  be 
still  +  0.63  volt.  After  again  cutting  out  the  30,000  ohms  for 
27  hours,  it  still  showed,  on  inserting  it  again,  +  0.434,  rising 
after  resting  a  few  minutes  to  -f  0.456  volt.  At  this  point, 
after  a  total  of  47  hours,  the  electrodes  were  cleaned  and 


TUB    ELECTROMOTIVE    FORCE    OF    METALS.  79 

weighed,  and  b  was  found  to  have  lost  24.06  mg.  and  o  to  have 
lost  only  13.25  mg.  Here,  again,  the  positive  current  has  moved 
through  the  solution  from  b  to  o,  and  more  gold  has  dissolved  in  the 
vessel  containing  no  oxidizer,  than  in  the  one  containing  the  oxidizer. 

In  some  other  experiments  with  peroxide  of  hydrogen, 
there  was  more  local  action  in  0,  and  the  o  strip  lost  as  much, 
and  in  some  cases  even  twice  as  much,  as  the  b  strip.  The  ex- 
act conditions  governing  this  local  action  are  still  under  inves- 
tigation. Bid  in  these  cases,  also,  the  galvanometer  showed  that  the 
positive  current  was  still  flowing  through  the  solution  from  the  strip 
b  to  the  strip  o  in  contact  with  the  cyanide  containing  the  oxidizer,  and 
thence  back  through  the  gold  strip  o  back  again  to  b,  the  place  of  be- 
ginning. 

The  course  of  the  negative  current  may  be  traced  from  the 
gold  strip  o  immersed  in  the  oxygenated  cyanide  to  the  strip  b 
immersed  in  the  unoxygenated  cyanide  in  two  ways,  as  follows  : 

1.  According  to  Ostwald*  the  reaction  02  +  H2  =  40H 
produces  4  X  21,100  calories.  Assuming  this  to  be  true,  the 
oxygen  molecule  O.2  forms  with  the  water  four  negative  hy- 
droxyl  ions,  4  (OH)  (  —  );  these,  assuming  a  negative  charge 
from  the  electrode  o,  cause  that  end  of  the  gold  electrode  to  be 
positively  electrified.  Now  these  negative  ions  travel  through 
the  solution,  displacing  at  the  other  end  of  the  line  four  nega- 
tively electrified  cyanogen  ions,  4  (Cy)  (  —  ),  which  give  up  their 
negative  charge  at  the  other  gold  electrode  6,  and  thus  enable 
four  positive  gold  ions,  4  Au  (+),  to  go  into  solution  there,  form- 
ing with  eight  cyanogen  ions  four  complex  negative  ions, 
4  (AuCy2)  (-). 

The  water  present  may  be  regarded  as  not  dissociated  ap- 
preciably, and  the  dilute  solution  of  cyanide  of  potassium  as 
entirely  so.  Making  these  assumptions,  the  principal  reactions 
may  be  expressed  as  follows  : 

O,  (±)  +  2H20  (=b)  +  4Au  (±)  +  8K  (+  )  +  8  (Cy)  (—  )  =  8K  (+) 
+  40H(—  )  +  4(AuCy.2)(—  ). 

But  this  is  equivalent  to  the  so-called  Ellsner  reaction  : 

O2  +  2H20  -f  4Au  +  8KCy  =  4KAuCy2  -f  4KOH, 
which  Maclaurinf  has  proved  to  be  quantitatively  correct. 


*  Chemische  Energie,  p.  956.  f  J™>r-  ^hem-  £°c->  voL  lxiii->  P-  728> 


80  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

2.  The  other  view,  following  Traube,  has  been  urged  by 
Bodlaender,  of  the  Clausthal  Bergakademie*  He  shows  first, 
in  agreement  with  Maclaurin  and  myself,  that  the  reaction 

H20  +  2Au  +  4KCy  =  2KAuCy2  -f  2KHO  +  H2, 

proposed  by  Macarthur  to  explain  the  solution  of  cyanide  of 
gold  in  cyanide  solutions,  is  incorrect.  Next,  he  claims  that  the 
so-called  Ellsner  reaction  really  proceeds  in  two  stages : 

(a)  The  hydrogen,  which  is  not  formed  according  to  Mac- 
arthur's  reaction,  is,  in  the  presence  of  cyanide  of  potassium, 
water,  gold  and  oxygen,  potentially  nascent;  and  a  molecule  of 
oxygen  combines  directly  with  two  atoms  of  nascent  hydrogen, 
forming  hydrogen-peroxide,  while  two  atoms  of  gold  dissolve ; 
— thus : 

02  +  2H20  +  2Au  +  4KCy  =  2KAuCy2  +  2KIIO  +  H202. 

(b)  Next,  the  hydrogen  peroxide  gradually  dissociates  into 
hydroxyl,  and  causes  the  solution  of  two  more  atoms  of  gold 
thus : 

H202  +  2Au  -f-  4KCy  =  2KAuCy2  -f  2KHO. 

The  sum  of  these  two  reactions  is,  of  course,  the  same  as 
that  of  the  Ellsner  reaction,  which  correctly  expresses  the  end- 
result. 

When  gold  was  rapidly  dissolved  in  an  aerated  cyanide  solu- 
tion, Bodlaender  was  able  to  detect  as  much  as  72.3  per  cent, 
of  the  hydrogen-peroxide  required  by  reaction  (a) ;  and,  as  re- 
action (b)  had  probably  already  set  in,  this  renders  his  explana- 
tion extremely  probable. 

Expressed  in  terms  of  the  ions,  reactions  (a)  and  (b)  become  : 

0)  02(±)  +  2H20(±)  +  2Au(±)  +.  4K(+)  +  4Cy(— )  =  4K(+)  + 
2AuCy2(— )  +  20H(— )  +  H202(±). 

(b)  H202(±)  +  2 Au(± )  +  4K(+)  +  4Cy(— )  =  4K(+)  +  2 AuCy2(— ) 

+  20H(— ). 

The  flow  of  ions  through  the  solution  is  the  same  as  in  the 

*  Ztitschr.  f.  angewandte  Chemie,  1896,  p.  583. 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  81 

first  case.     On  the  whole,  the  second  seems  the  more  probable 
explanation,  though  either  agrees  with  most  of  the  facts.* 

According  to  either  of  these  views  the  new  theory  agrees 
quantitatively  with  the  results  of  experiment,  but  offers  for  the 
first  time  a  consistent  explanation  of  its  occurrence.  It  is  due 
to  the  superior  electromotive  force  of  the  oxygen  (or,  in  case  they  are 
present,  to  some  other  electronegative  ions,  as  (OH)  ( — ),  Cl( — ) 
Br  ( — ),  etc.),  together  with  the  capacity  of  the  gold  for  forming  com- 
plex ions  with  cyanogen. 

*  While  this  paper  was  in  press,  a  paper  on  "  Freiwillige  Oxydation  "  (Auto- 
oxidation),  by  Dr.  Manchot,  of  Goettingen,  has  appeared,  in  which  he  has  ex- 
amined the  oxidation  of  a  large  number  of  phenol-derivatives,  such  as  those  used 
as  developers  in  photography.  One  of  these  derivatives,  oxanthranol,  was  par- 
ticularly well  adapted  to  give  quantitative  results,  and  he  was  able  to  prove  that 
for  every  molecule  of  oxygen  absorbed  a  molecule  of  hydrogen  in  the  oxanthranol 
was  oxidized,  and  a  molecule  of  hydrogen-peroxide  was  formed. 

^Representing  the  organic  radical  by  K,  and  the  oxanthranol  by  RH2,  he  assumes 
that  the  reaction  takes  place  as  follows  : 

KH2  +  O2  =  E  -f  H2O2. 

The  organic  radical,  if  unstable,  is  frequently  still  further  oxidized  in  a  second 
reaction  by  the  hydrogen-peroxide  thus  formed. 

It  would  appear  that  similar  reactions  ensue  in  the  rusting  of  metals  in  damp 
air.  The  rusting  of  iron,  zinc,  etc.,  is  worthy  of  thorough  study  in  the  light  of 
these  new  ideas. 

It  would  seem  that  the  modern  electrochemical  views  necessitate  a  return,  in 
part  at  least,  to  the  ideas  of  Berzelius  and  Schoenbein.  They  supposed  that  the 
same  element  was  at  times  positively,  and  at  other  times  negatively,  electrified. 
This  appears  to  be  a  consequence  of  the  new  view  also.  For  if  we  regard  the 
oxygen  molecule  O2fdr)  as  electrically  neutral,  this  can  only  be  the  case  when  one 
of  its  atoms  has  a  double  positive  and  the  other  an  equal  negative  charge.  By 
the  attraction  of  these  charges  the  molecule  may  be  regarded  as  being  held  to- 
gether. Its  real  composition  then  would  be  O( )  -\-  O( +-}-)•  O°  tne  other 

hand,  two  atoms  of  oxygen  in  the  elemental  state  would  be  similarly  electrified 

with  negative  electricity,  thus  :  O( ),  O( ),  and  would  consequently  repel 

each  other.     Hence,  to  change  an  oxygen  molecule  into  two  oxygen  atoms  would 
require  four  units  of  negative  electricity. 

On  the  other  hand,  the  hydrogen  molecule  would  be  composed  as  follows  : 
H'(-f)  -f  H(— )  ;  and  to  change  it  into  two  hydrogen  atoms  H(-j-)  and  H(-J-) 
would  require  two  units  of  positive  electricity. 

It  would  also  seem  necessary  to  assume  that  there  is  an  inherent  tendency  in 
the  oxygen  molecule  (due,  perhaps,  to  some  peculiarity  of  shape  or  volume)  to 
assume  negative,  and  in  the  hydrogen  molecule  to  assume  positive,  electricity  in 
dissociating. 

It  would  also  appear  as  if  a  different  result  ought  to  be  produced  when  neutral 
hydrogen  molecules  combine  with  a  neutral  oxygen  molecule,  from  that  which 
results  from  the  combination  of  positively  electrified  hydrogen  atoms  with  a  neu- 
tral oxygen  molecule.  This  may  be  the  key  to  the  formation  of  water  in  the  one 
case  and  hydrogen-peroxide  in  the  other. 

6 


82  THE    ELECTROMOTIVE    FORCE    OF    METALS. 

If  instead  of  having  the  two  ends  of  the  gold  strip  immersed 
in  two  separate  cyanide  solutions,  the  strip  is  immersed  in  the 
same  solution  containing  some  dissolved  oxygen,  the  same  elec- 
trolytic action  can  still  go  on  as  a  case  of  "  local  action;"  for 
the  couple 

Au  :  KCy 
(OH)  :~Au 

is  still  possible  if  we  regard  the  gold  to  be  short-circuited  on 
itself,  and  the  explanation  given  above  still  applies. 

When  I  began  this  investigation  in  1896,  I  marked  out  for 
myself  a  much  wider  range  of  investigation  than  here  outlined, 
and  the  course  of  its  partial  execution  has  suggested  many 
other  interesting  questions,  some  of  which  are  still  under  inves- 
tigation ;  but  the  constant  and  pressing  interruptions  of  routine- 
work  have  made  it  impossible  to  carry  the  work  further  at  the 
present  time. 

IY. — CONCLUSIONS. 

Whatever  may  be  the  nature  of  the  objections  that  may  be 
raised  against  the  final  acceptance  of  the  modern  electrolytic 
theory  in  its  present  form,  it  will,  I  think,  be  conceded  that  the 
following  conclusions  may  be  fairly  drawn  from  the  foregoing : 

1.  That  the  new  electrolytic  theory  explains  in  a  remarkably 
complete  manner  the  reason  for  the  departure  of  metals  im- 
mersed in  cyanide  solutions  from  the  sequence  of  electromo- 
tive force  which  they  present  in  acid  solutions. 

2.  That  it  is  the  only  theory  ever  presented  that  gives  any  clue 
to  the  remarkable  aberration  of  cyanide  solutions  from  all  the 
usual  chemical  analogies. 

3.  That  it  explains  in  an  entirely  adequate  manner  the  rea- 
son for  the  reactions  that  go  on  when  gold,  silver  and  other 
metals  are  dissolved  and  precipitated  from  cyanide  solutions. 

4.  That  the  determination  of  the  electromotive  force  of  the 
metals  in  cyanide  solutions  under  different  conditions  offers  a 
means  of  research  that  is  likely  to  be  of  great  practical  utility 
in  determining  the  direction   and  intensity  of  chemical  reac- 
tions, under  fixed  conditions,  or  in  following  them  under  chang- 
ing conditions,  just  as  they  occur. 

5.  That  the  differences  of  electromotive  force  of  metals  in  di- 
lute cyanide  solutions  do  not  give  much  support  to  the  so-called 


THE    ELECTROMOTIVE    FORCE    OF    METALS.  83 

"  selective  affinity  of  dilute  cyanide  solutions  for  gold,"  the  only 
common  metal  that  shows  any  indication  of  such  favorable  ac- 
tion being  copper. 

6.  It  is  probable  that,  in  the  absence  of  external  electromo- 
tive forces,  an  aerated  cyanide  solution  less  than  or 

10,000 

0.00065  per  cent,  is  without  action  on  metallic  gold. 

7.  That  for  all  practical  purposes,  an  aerated  cyanide  solu- 
tion of  less  than  0.001  per  cent,  is  without  action  on  metallic 
gold. 

This  study  has  led  apparently  far  afield  from  the  practical 
side  of  the  cyanide  process ;  yet  I  hope  that  it  may  be  of  service 
in  at  least  calling  attention  to  the  work  of  others  who  have 
toiled  for  many  years  in  attempting  to  clear  up  some  of  the 
most  subtle  questions  that  have  ever  taxed  the  human  mind. 
For  I  am  firmly  convinced  that,  in  the  long  run,  such  work  is 
always  of  the  greatest  practical  service.  In  these  days,  the 
words  of  Ostwald  have  certainly  come  true  : 

"  The  science  of  to-day  is  the  practice  of  to-morrowT." 


•I":*:"  !**'•::  A 


SEP.51975    8 


LD21 


_100m-9,'47(A5702sl6)476 


YC   18748 


321305 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


